I try setting this to 9999 but the squad didn't really leech any noticeable health...
You must also make sure that the weapons of the squad (those that you want to leech health with each attack), ALREADY have life_leech_damage > 0. (Look into the armour_damage of the weapon for this)
Because if the leech damage is 0, even multiplying it by 10^100, leeching will still be .... Zero!
Now, if you want the leech to work AFTER a research, then you can either set leeching to 0.01 (so that it is not initially noticeable) and set a big multiplication modifier afterwards in the research.
OR ... Simply use Addition.
I tried a custom weapon,give it life leech as 1 and modifier is 9999 but it seem like there are no noticeable difference.Should i increase the life leech more ?