We have a concern, we just finished putting the backer board up in our shower and found where the wall meets it is not square. It looks good at bottom corner but as it goes up the back wall must be set back some. The gap in the corner starts from nothing to about an inch at ceiling. Should we take the cement board off and build it out somehow? I was told to just fill it in with the thinset but that won't square the tiles as i set them outwards from the corner. Maybe it will all work out just want to make sure before I move forward. Thanks for your time!