I use Duramectin, and the dosage is 1 click of the syringe for every 50 lbs of body weight. So for a full dose, I do that, rounded up. So if you're 180 lbs, do 4 clicks (for 200 lbs). For the half doses, I just halve it, rounding up. So if a full dose is 150 lbs (3 clicks), a half dose would be 2 clicks (100 lbs). When I feel I'm getting sick, I do a full dose on day 1, along with some Quercetin and zinc, and then do that again 2 days later (even if I'm feeling better). I do half doses about every 3 days, along with topical application, just for my rosacea. That seems to be what works best for me for that purpose. Other people probably have a different regimen for prophylactic use. My father takes a full dose once a week and it keeps his allergic asthma under control.