Dude, best way to know is to try. I use a (fake cigarette) one hitter and I pack that 3/4 of the way and that give me two full hits. Gets me exactly where I want to be.

Others may need to do that 4, 5, or 10 times.

best thing to do is take two puffs and wait 10 minutes. Take one LAST hit and wait 10 minutes. Didn't get high? Don't worry, it was your first time. Next day try that again, two hits and wait 10-15 minutes and see how much you need to get to where you want to be. Make sense?