I have always used Vet RX for chickens, straight from the bottle, rub liberally on feet and legs right into the feathers above the legs. I use it spring, summer and fall (aprox. once every other month) when migrant birds come through to prevent ever getting leg mites. I have never had a problem with leg mites doing it this way. I know it works great. I used it on two chickens years back that I got from a friend who had them pretty bad. Applied it each night for a week and they were gone.
It is also good if you hear any chicken sneeze in the cold season. I put it on my hands and wipe their chest and under each wing (only the excess that is on your hands) and it seems to clear them up over night.