That's fine for now, i'll judge your pool only by profits made, but your reasonment here have a great bias: the variable here is the fees income, not the server cost. If your pools perform better than expected you could set LOWER fees, while in "bad days" you could need HIGHER fees to mantain the servers.
If you otherwise decided to set variable fees in the way you said for marketing reason it's more understandable. But, either ways, i think you should asap declare how much the fees actually are. No way to test if you're saying the truth since we don't know what coin we're mining so we can't do a accurate estimate of the expected profit (just some guesses based on net difficulty), but knowing how much fees you're paying make the user much more confortable.

PS: me too think that multipool owners are at most cheaters or incompetent. Never find a decent multipool till now (i.e. better than mining doge or whatever on a single pool with ~1%fees, switching coin every 1/2days), i really hope that yours is different