Do an alcohol wash. I feel it is much easier and whole lot less time consuming.
1. Pick a frame with some capped and older open brood. This frame will contain nurse bees which is the age you want to test.
2. Be sure the queen is not on the frame and shake the bees into your top cover for your hive.
3. Replace the frame in the hive.
4. Pick up the top cover, which now has alot of bees in it and shake them to the corner of the cover and fill a 1/2 cup measuring cup.
5. Dump the bees from the measuring cup into a jar with some alcohol, already measured and set aside, and place the lid on the jar.
6. Shake the bees and alcohol for a minute or so and dump thru a collander, to hold the bees, and thru some white filtering cloth, to stop the mites into a bucket.
7. Set the jar down and count the mites.
This whole process is alot easier to do than to explain. I might add it can be seen on YouTube thru NOD Apiaries videos. Once you are set up to do it a lot of hives can be done in a short amount of time. Also I might add there is absolutely no need for a funnel or other devices and the 1/2 cup when full will contain about 150 bees to give you a ratio.