i think a % can work if you making it a scaling percentage. so as you increase your max WI you can recruit a higher number of creatures. maybe that 30% would go down to 25, then 20, then 15, then 10, so it still rewards stat progression.
if you have millions of WI to start with, making it a flat 30% is just going to feel punishing for having put in all that effort to get there and remove all sense of acheivement for putting in the grind. if it's 30% plus a flat number, then you can only ever recruit 3 creatures , however, if you have mililons of WI, it's going to take you MUCH longer to regenerate that 30% to be able to recruit another, than say a newbie with barely any. the solution to that would be to make WI regerenate as a % , but then again, then tha tjust removes the point of farming WI to begin with, and it might as well be the same fixed value for everyone