Jeets with your figures at 1,500 bees per frame and 20 frames you would have 30,000 bees in that hive. Divide 30,000 by 100 then 1% would be 300 mites. But only 15 dropped divide 15 by 300 and your phoretic mite count would be 0.05% but as you have brood and that 0.05% is only 1/5 of the mites you can multiply by 5 and the total mite infestation of the hive would be 0.25%. This will vary with the amount of full frames of bees in the hive. for full medium frames I estimate at 1000 bees per frame.