Shit. Duh. Basically use a similar trick for the value of oldM as the second half of the newM equation. Wthin a range large enough to be painful but not so large as to be obscene, say 30%, hard code an inverse relationship between the total amount of oldM that has not been withdrawn yet and how much xmr you receive for your oldM. You would receive less xmr for your oldM the sooner you withdrew. There are plenty abandoned accounts I'm sure and some large holders locked out so the full range of variance would never be realized. But people might slow their roll a bit when it comes to getting in line to withdraw if they knew if they waited longer they could get their full amount rather than a 5% haircut. It might be possible actually let people withdraw MORE xmr than they put in if the waited long enough... if the game is getting used and generating the inverse-crackhead-fractional-nonsense-stash. Might border on ponzi scheme though. Have to play with the numbers.