This is not unreasonable, but I'm not certain it would be all that great to keep a 1:1 ratio forever and I am somewhat concerned about the potential for abuse. For this season (post-covid, extra eligibility, open transfers), I'd be down with one exempt transfer in for every transfer out, but I think I would want some upper limit moving forward. Something like a 1:1 exemption ratio for up to 5 transfers. Beyond that, you get no additional IC relief for transfer. I'd also want to make certain transfer ICs could only be used for transfers (not HS kids, otherwise we aren't solving the problem of getting these kids out of the portal).
It would also make sense to allow each graduate transfer out to allow for an IC exemption beyond any proposed limit. So let's say UM had 8 kids transfer out, but 2 graduated, UM would get 7 ICs for transfer beyond their 25 ICs (limit of 5 plus 2 more for the graduate transfers). This way the schools who are graduating kids before they transfer get a little more IC relief than the schools who are just pushing kids out to make more room for transfers.