I've tried a few methods and still came up wanting, till now. My problem john had a flange that stuck a little too high above the floor and was less than level as well. Aside from being annoying it ate wax rings quickly. My solution; I patterned the base of the toilet, and cut a piece of smoked plexiglass roughly 1/2" bigger than the base (still a rectangle, I didn't attempt to contour it precisely), rounded the corners and cut a 7" hole for the flange in the appropriate spot. It cracked on the one side of the big hole, but as I was siliconing it down, I didn't let that bother me. That 1/8" made all the difference, it is rock solid, and even seems to work better, and the smoked plexiglass actually looks pretty good, unusual perhaps, but good.