I am installing a back-outlet toilet as part of a remodelling project. The toilet is a Kohler K-3554-0 model. I currently have a (capped) 3-inch PVC DWV pipe protruding from the wall and it is finally time to select a flange and gasket for installing the toilet.

For the flange, I am considering either the 888-P or the 888-GPM from Sioux Chief. Also, once I select the flange, what is the best way to support it? The pipe receiving the flange is anchored fairly well has very little "give" in any direction, but I am still sorely tempted to build a 2x4 cage for it anyway. I tend to over-engineer everything and I would appreciate some suggestions before I blow this out of proportion.

The other concern I have is the centerline of the DWV pipe. My understanding is that missing the centerline on these things is a very bad thing. The Kohler guide calls for a 4-inch rough-in height. The stone floor coming out met this requirement, but the replacement porcelain tile is substantially thinner (about 7/16"). Is there anything I can use as a "shim beneath the toilet to compensate for the differential that will still look decent?