Basically, you take as little as you need.
I had some 100 mg, to take for help with sex on e (on the comedown). I started out taking half, which had strong effects, and then started taking smaller pieces, like 1/3 and still got the effects though maybe a bit weaker. Now that I see they make 25mg doses (I wasn't aware of that) I'll probably try 1/4 next time.
For girls, I thought they weren't supposed to take it. I know it has been said to have positive sexual effects for women, but isn't there some danger since it is designed for men? I know girls who've enjoyed taking it, but it seems risky to me. Anyway, I doubt you will find any good guidelines for how much a girl should take, but I'd guess she should do the same: 25 mg to start.
I think taking a whole 100mg would be major overkill for anyone, particularly since you are probably looking to mix this with other drugs which can be risky and hence prudence dictates taking smaller amounts.
~psychoblast~