If I read this right you are going to put like 1/2" drywall behind the stool, or some other wall covering and you are worried about the stool fitting on. Would it not be easier to get the 1/2" back from the wall you are covering??? Like if you are putting drywall over lath and plaster(done all the time here). Just take the lath and plaster OFF that one wall, chances are its a short one anyway. Easier than trying to move a second story pipe flange 1/2".
BUT if you cant do that, what type of drain pipe is is, cast?plastic? lead? Most will give 1/2" if you HAVE to!!!
While you are at it put some resiliant wall covering over the back wall so you dont have to paint behind a stool, nothing worse than that job and nothing ever fits anyway. Its a good place to practice your ceramic tile laying!!LOL--don--