Well no, there is a fixed percentage that determines Max Leech, then the leech on your weapon determines which part of that Max Leech you actually get.
So for each hit you'll receive [Weapon Leech]% of [Max Leech].
Your Max Leech is : Damage * [fixed value]
Your actual leech is for example : 0,6 * Max Leech. (if your weapon has 60% Mana Leech)
I hope it makes sense... I'm a bit tired, I guess it shows
*edit*
HAHA found it !
Stratics Forums Archive: Re: List of Changes on TC9 - Pub42