I guess the variable numbers for acceptable varroa count are putting people off offering one. Also I know TF often means not checking as makes no difference.
Found this site and I think we have a slightly colder winter but rest is similar
http://extension.msstate.edu/publications/managing-varroa-mites-honey-bee-colonies Leave the sticky board in place for 3 days, and then count the number of dead mites. The daily ?mite drop? is estimated by dividing the total mites by the number of days for the sampling period. The economic thresholds estimated for using this technique vary widely from region to region. However, a reasonably conservative threshold for late summer in Mississippi would be a daily mite drop greater than 40?50 mites per day.
So as it is what I want to hear I will go with it, my 21/day is ok then. I have seen these as low as 2/day and recommending treatment, seems way off. Also 100/day before worrying, seems too high.
If mine continue at 20-30/day I won't worry about them, all I was planning was sugar drifting from the top between the bars but not bothering tem sounds better.
Sticky board is my only option as I won't be harvesting bees for testing, this is just my preferred method and should be informative enough for my purposes.
Adding these doesen't disturb the hives so I can do as many as I like.