I was replacing my toilet and found that my flange was rusted so much that I couldn't get the flange bolt on one side to lock into the flange so I must replace the flange. It's made of metal with ABS and obviously is glued into the ABS sewer pipe riser. I'm assuming that I need to get underneath the bathroom floor, cut the riser pipe and pull out the flange with pipe attached, glue on a coupler and then attach a new flange to an appropriate length of ABS to glue into the other end of the coupler. Where should my ABS pipe stub up into the bathroom floor? Floor level, 1" below, 2" below? I've never done this but I have worked a little with ABS and PVC pipe. Thanks for any help and advice you might have.