Personally it isn't as simple as a wash, although it's a good indicator. I combine it with uncapping late capped drones, no real count but it helps to give me a better idea of population levels. If I get 2 mites in a shake/roll and I find most drone brood to have mites then it's absolute treatment. 5 Mites all by itself would inspire me to treat espcially if I saw even 1 virus infected bee.
Remember when the drones get booted out, soon, the colony inherets all the mite load they promoted and nursed so just testing the bees can be misleading, although part of the picture. Test 3-4 times through the year, log it and look for trends in colonies and in yards. This really helps to make decisions, keep colony cards for many factors in addition to mites. I like formic acid, kills both varroa and tracheal mites, organic certified, and it kills at least some of the mites in cells which oxalic doesn't do.
This time of year, as the populations of our colonies is in decline, you want to keep the mite load under 2%, according to the Honey Bee Health Coalition. That means anything over 6 mites per 300 bees calls for some kind of intervention. But you also need to do regular tests this time of year, because the mite loads tend to skyrocket as the bee populations decrease in size. I mean, I've seen seen huge jumps in mite loads over just a few weeks early in the fall, so you've really got to be on top of it.
Nationwide in the United States, August 15th (or as close to it as you can) is GIANT MITE BOMB DAY. This is the one day of the year that IPM (Integrated Pest Management - the sparing use of pest controls) adherents will throw a big nasty weapon at the varroa mites because, while it will take it's toll, there is still time for your bees to recover and prepare for Winter.
HINT: Use formic acid, usually purchased by beekeepers as mite-Away Quick Strips, MAQS, for short. This stuff is [email protected]$$, and properly applied it will kill 100% of mites in a beehive, whether ectopic or under the brood caps.
True, bees hate the stuff. Queens suffer (remove the queen for a day or two during treatment if your only have a few colonies ((or a prized queen or two or three)), then return them to the exact same hive!) considerably under formic acid, so you'll need to be ready to re-queen soon afterwards if you expose them to it.
So, to answer your question, after treatment, that is, ZERO is a tolerable mite count in late August. If you have not yet treated, do so using MAQS or other formic acid treatment (I use the stuff straight out of a large container of the nasty stuff, soaked onto blue auto shop paper towels). It goes on the top of the highest frames in the hive (don't have to remove honey boxes) because the stuff goes DOWN in the hive. Wait a week, then do an alcohol wash and check that you indeed have zero mites.
On a similar tune, treating ALL your bees in a drop over the same 2-day stretch tends to remove ALL the mites in that apiary. Good for Wintering.