Oh, goodness, forty mites between April 12 - May 3? That's not bad (40/12= 3.3) , so during that period you didn't have to worry. But it's the 14th of May and who knows what's going on now? Time to test again!
You have an excellent set-up, similar to mine from Betterbee.
The first thing to do is make sure your hive stack is arranged so that the board slot opens towards the back of the hive. This allows you to pull the boards whenever you feel like, even at night when messing around at the entrance might make the girls antsy.
I see you have a board with dark grid on it. That may make seeing the mites harder on the dark lines. My boards are just plain, which works well. (And you can sometimes score free material to make boards after an election season when there are discarded political signs along the road. These have many uses in an apiary, so don't pass any up. If yours have printing on them they can be sprayed with white pigmented shellac and turned into perfect mite monitoring surfaces.)
It's helpful to have at least two boards per hive, because then you can stick a fresh one in at the same time you pull the current one out, so you can always be testing, if you want. (And there is stuff to see, besides mites, on the boards from the patterns of debris, evidence of SHB larva, etc.) But even if you don't have two boards per hive, I'd recommend a dedicated one for each hive because then you can pull them all at once and carry them indoors for counting and if they are each marked with a unique hive number, name, or some other code you won't confuse one set of results with the other. And the hives will vary, so expect that.
Also, I assume the the Brushy Mountain board has a wooden piece, or other cover for the slot? If not make one so that no bees can get into the chamber below the screen. That could skew the counts
Before you use the board take a large paperclip and un-bend it enough and poke it through a hole made about 1/2-3/8ths inch in from the edge of one of the short sides, about halfway across from side to side. Work the paperclip around so it becomes a sort of flippy tab sticking off the end of the board, rebending the first segment as needed. This makes a convenient way to grab the edge of the board to pull it out, even with gloved fingers.
So, starting with a clean board, pour about a tablespoon of olive (or other kind of vegetable) oil and spread it around, evenly over the surface of the board with a paper towel. Repeat for each additional board, but use the same towel and about 1/3 or 1/2 the oil on subsequent boards as the towel soaks up a lot on the first one.
Take the board out and slot it in. Study the positioning so that the board covers the entire bottom surface where the mites can fall. Depending on the design of the equipment it may be possible for the board to extend under the solid part of the entrance where mites can't fall, exposing part of the bottom at the back where they can fall. You only have to do this once, and you can mark your bottom board so you can get it right ever afterward.
Go away, and come back in pretty much 72 hours to remove the board. I try to do it within an hour or two of the 72-hour period. If that's impossible and the interval lengthens (or is unavoidably shortened) by more than four hours, I will use the appropriate fraction of day when calculating average mites/day. I also try to run tests when I am not likely to be in the hives for other reasons. So, if you are mostly a weekend beekeeper, I'd run the tests sometime during Mon.-Fri., instead. If your weeknight schedule means you need to put the boards in during the evening, the bees won't even notice as long as the slot is on the back of the hive. It's a rare beekeeping chore that can even be done in the dark. (Well, except for counting, that is - you need a good light for that!)
I try to count immediately, but a board can wait in a protected place for a bit. I have left boards with known mites on them - even semi-live looking ones - and they don't go anywhere.
To count I use a magnifying glass, a small LED flashlight and a slightly smashed-end, damp wooden toothpick. I hold the glass in my left hand, the toothpick in my right and the flashlight in my mouth.
I start anywhere on the board and every time I see a mite, I pick it up with the tip of the toothpick (that's why flattened and damp works best) and set it in a clear place on the board. And then go on to find the next one. Picking them up and setting them aside to be counted makes the scan fast and easy, with no double counting, or fatigue from a hard scanning pattern. You basically keep looking until you stop finding mites. You will get much better at it, the more times you do it. You will also get a kind visual mite-sense about the shape of them when they are not lying flat, but tipped on their sides. Not all mites are the same color. Males, and some juveniles are lighter, almost the color of ginger ale. I remove and count those as well, though I know some people don't.
When you don't see any more mites, use the glass to look over your collection and discard anything that's NOT a mite, which at first may be quite a few little brown specks. Sort the mite into goups of five, count them and divide by the number days and you're done. Except for one thing: write the number down. You may think you'll always remember, but you won't. And with passive mite fall monitoring it's the change over time thats what you're testing for. So if you don't have good records of every test, you are wasting most of the gain you'll get from testing.
After you've done the count, just scrape off the rest of the debris (into a closed garbage can if you have SHB larva on the boards - don't let those puppies reach the soil to pupate!), wash the board in clean, warm water and dish soap, let dry until you retest.
Now continuous monitoring maybe be too much information for your style of beekeeping, but doing a 3-day count once per week in your first summer and through fall wouldn't be a bad plan, as there is not too much else to do, and your interest is still very high and you will gain a satisfying sense of being a competent bee "keeper" not just a bee "haver". There is so much to learn the first year that it can feel daunting. But you can EASILY learn to do sticky-boarding, so I think it is good counter balance to the first-year beekeeper's feeling of being overwhelmed. Plus, and this you will find very satisfying to be able to say, "Why, yes, I do know my mite counts, and last week they were X, and compared to a three weeks ago, that is up/down." Most new beekeepers first grapple with mites only when they begin to see a problem and then feel they are behind the curve when asked this inevitable question. But if you've been testing all along, you will be on top of things. (And truthfully, far ahead of many beekeepers with years of experience who still seem to think they will "see" the mite on the bees if they have a mite problem. By the time you have a high enough infestation rate to easily see mites on your bees as they come and go, I think you're really in the weeds, and your colony is at great risk.)
No matter what you decide to do about treating - even doing nothing - knowing your mite counts tells you something that is important to know about what your bees are dealing with.
Now, what if you see a sudden spike - a day you do pull the boards, and there are dozens and dozens? Don't panic! Just count, wash the board and immediately retest, pulling the board every 24 hours for a quick, on-going, check. A few times (and I pulled boards every day for most of last year) I saw sharp rises that were not repeated. I imgaine that there was some corresponding change in brood hatching (particularly drone brood) patterns, or population or occupational shift among the bees. If the next successive counts go back to the previous levels, I wouldn't worry about it. But a sustained rise, particularly in the late summer/early fall when the bee population is starting to fall naturally and the mite population is starting to accelerate you need to pay attention to any rise in mite numbers. This is very important because at that time your hive is beginning to raise its over-wintering bees and these must be particularly strong and healthy to get through the long, cold, months to begin next year's build-up. And since mites predominantly predate the pupating bees, you want to do what you can to protect those nascent winter bees by lowering mite levels in time to have a positive effect on their early lives.
I hope this info is useful to you. And because you have already begun to run some tests, I think you are well on your way to being a successful beekeeper.
If you have more questions about mite testing and interpreting the levels, you might find the NYBeeWellness site (usually in box on the right of these pages) is very helpful, and geared for the northeast's climate as well.
Hope you have as much fun with your bees as I did in my first year, despite all my stressing over all the things I didn't have a clue about - my bees got me through it.