Completely disagree.

What happens if you do the math on child care costs for multiple kids, and decide someone's staying home while the other one works? They just don't get to buy anything? Or if there's a dramatic swing in earnings? "Sorry honey, I don't think you can afford this vacation, I'll see you when I get back".

I'm not suggesting planned "fun money" accounts/allowances are bad, I'm saying basing them off your earnings is a pretty crappy way to do it if you're a team.