I have a cast iron toilet flange that sits about .4 inch above a 3/4" plywood subfloor. It feels very solid, but it is not anchored to anything. There are two empty holes in the flange for attaching to the floor, at 11 o'clock and 4 o'clock positions. The plywood underneath the flange is water damaged and brittle but still fairly strong. I plan to install 1/4" cement board and tile. Should I bring the cement board under the flange and screw down my two holes all the way to the brittle plywood? There's not enough room to get the tile under there too. Could I use the Superior Tool super ring replacement flange, or the Oatey galvanized wood floor support? Thanks!
If your subfloor is questionable, you should tear up the section surrounding the flange, between the joists, then block between the joists on either side (or front and back, depending on orientation) of the flange, and replace the subfloor material. Then you can screw the flange to the new subfloor, with a spacer or shims.
You don't need the CBU or tile to fit under the flange.