I no longer treat, but when I did, the last method I used was OA vaporizing (the only thing I found to consistently work, though I never tried formic because it was not readily available at the time) . The first year I started, my hives where just about ready to crash so I did 3 treatments 1 week part in the Fall. For the 3 years after, I did one treatment a year in November and never had a varroa problem. Key is to monitor and be prepared to treat if you need to.
I did two treatments a week apart this fall and mite drop went from too numerous to count down to about 35/24 hours - about a month later. I used 1 gram of OA per treatment, and I suspect that I should have used a larger dosage for better results.
Good how-to and dosage info is hard to find. How much would you
use if you were going to do it again?