Hi
There are two methods I apply for varroa count to determine the % presence.
Both application come from research persons.
With the construct of a sreened bottomboard and a catch plate
beneath ,which is sprayed with food grade oil the normal daily "Drop off"
should not exceed 30 mites.
Percentage : Collect 100 bees in a jar , pour rubbing alcohol over them and
shake the jar ( Close the LID !)vigorously.
Pour the  alcohol and bees into a coarse honey screen . Have the fine
screen neneath it. Catch the alcohol in another vessel to be used again.
The varroa will fall through the coarse screen with the aid of a strong
water stream  over the bees into the lower one.Count the varroa and bees
that you get the percentage. Do this procedure again  and try to collect
the bees from another spot in the hive.
This will give you a fair percentage number.Another way of getting some
idea is the count of dead mites after chemical application on the bottom
board . You will note a demishing number after x days until there is no
more drop off. Whatever you do , go according to instruction as supplied by
the manufacturer and good practice.The advertised method with powder sugar
proved to me worthless.
However ---- We try to get away from applying chemicals to control Varroa
as well as other diseases.
Good luck
Juan de Fuca