I also have the plastic flange screwed to the top of the floor. We just replaced the entire floor, new flange, new toilet. Two weeks later the toilet is leaking again. I have not noticed any rocking, but there is a very small gap between the toilet base and the finished floor. Is our flange too high? Would buying the spacers you mentioned help? I am starting to buy wax rings more often than water filters! Please help.

If you can see a gap between the base and the floor then your flange is too high, just how high is it? If the gap isn't too high you could install a wax seal with a funnel and shim the bowl up and caulk it. Or you could cut the old flange out and install one next to the floor.

Would buying the spacers you mentioned help? No! The flange extenders are for flanges that are too low. The bowl allows for some "wiggle room" Exactly how high is it from the floor? Back to you, Tom