merk upjohn anticipated that you would just be putting it on the back when they said 1mL, but now we know to put it everywhere. i personally use 3-4 per session, but am trying to get down to 2mL to cut costs, or dilute with rubbing alcohol to spread more and get higher penetration (according to bryan).
some people on here are affraid you'd get to much systemically if you use more than 1mL. use using more does mean more systemically, the fact is most people can handle xandrox 15%, so i think you will be fine.
i can't get 1 mL to cover my whole scalp. some people say their box says that if they place drops every half or inch, i forget which, I thought they said every inch, the minoxidil will diffuse under the skin to get the other areas. however, Bryan says dr proctor does not believe that occurs much.
if you are just using minoxidil and propecia, i think you should use as many mL of 5% as needed to get everything covered, though not drownded.
bryan says the propylene glycol is mainly to dissolve more than 2.9% minoxidil to make the 5%, and is not needed to increase absorption, because optiomal absorption occurs with 90% alcohol. I don't know if the propylene glycol counts as an alcohol here, but rogaine does have 15% water in it. perhaps that is so your skin does not burn as much.
i'm going to dilute my 5% to 3.5% for more coverage. I just wish i knew how much the water in the alcohol is a problem, and am going to look into perfumer's alcohol, though i don't want any methanol.