that's actually exactly what it is based on already
you pay less if she makes more... you pay less if you have more visitation... you pay less if you make less
it's all for the kids.... so if combined you make 100k... the kid will receive lets say 2k a month
now if that's you making 100k and her making 0.... well your fault for knocking up a bum bytch... but you paying the 2 yourself
if you making 50 and she making 50.... then maybe it'll be 1k... she'll make up the other 1k they WOULD have made you pay
if you have shared custody, nobody pays the other... you both responsible for it on your own... if she has the kid everyday except 4 out of the month...... well......