I am switching Sonny from his current flea treatment to Comfortis and I don't know which dosage to buy for him. His weight currently fluctuates between 19.5-20.5 lbs. I am not sure whether to buy him the dosage for 10-20 lb dogs or the one for 20-40 lb dogs. I want to give him a strong enough dosage for the treatment to work, but I also don't want to over treat him. I'm stumped....Any advice is greatly appreciated! Thanks!