When I was counting mites, I would do three things. Mostly because none of them are 100% reliable. They are affected by temperatures and other factors that are hard to pinpoint. Drones can be heavily infested in one location in the hive and not in another. So if you uncap some drones, do a sugar shake and do a natural drop count, you have some redundancy built in. Also, I think the TREND is more important than the number. If the numbers are rapidly growing you have more of an issue than if they are staying steady, no matter what those numbers might be.
Great point Michael, always watch the trend not the count. If you use a computer setup a spreadsheet with a graph, record counts over the year and see when you have a build ups.
Note on graph when you treat (if you do treat) and see if the trend goes down? Don't wait until you can see mites walking all over the comb.