When I used to flash treat with FA I used 80 mils of 50% FA for a deep
so about 30mils should be OK for the O/ hive. I put the 50%FA into a little bowl and soaked the acid into a cotton ball and squeezed the excess out before putting it on the top screen, you do not want the acid to drip out onto the bees and I did 2 cotton balls in this way. Since that time I have made the band heater type of OA vaporizer and I have made a 1/4" hole in the side just above the lower frame which is plugged with a little 1/4" dowel so now I just insert the 3/16' outlet into the hole and vaporize with a little OA which is less disruptive to the little colony. I have found that FA use is never consistent as temperature plays a major part and the temp cannot really be controlled as just the amount of bees in a small area with a lot of brood can make a difference.
Johno