Well before you worry about the concrete, need to make sure you get
the rough measurements right. Measure 15" off the wall beside the
toilet. Make sure you allow for FINISHED WALL (add 1/2" if your coming
back with sheetrock...add closer to a full 1-inch if your coming back
to do the walls with tile.
That said, code only requires a 3" standpipe for that toilet. Stub the
pipe up high enough to pour around it. Use some insulation and wrap it
around the pipe (you want about 4-1/2 inches so the flange will fit).
Tape the insulation with duct tape...pour your concrete...after it
hardens, pull the insulation out and you'll have room for your flange.
Swimming Floaties also work well (the thing kids wear on the arms at
the swimming pool). You can use the larger sized. Inflated, they are
the perfect size...and you can use them numerous times.