I am working on finishing my basement bath (concrete floor) and I set the closet flange today (PVC with a stainless steel ring). The problem is that the flange does not sit perfectly flat on the floor, it is flush in the front but the back is raised up about 3/16 of an inch (it is however level side to side). When I grab onto the flange and try and move it around it is rock solid. I'm thinking that the reason is because of the 1/4 per foot slope on the waste lines. But being that the pipes are encased in concrete there is absolutely no give there to get it flat. I put my wife’s best fire engine red lipstick on the high side of the flange set the toilet on top and wiggled it around. There was no transfer to the bottom of the toilet, so at least it's not holding the toilet up off the floor. My thinking is to just put some hard plastic shims under the flange and set the toilet in wax and hope for the best. I would appreciate any input from you guys... thanks for you time!