I hesitate to bring this point up but i think guess your walls are out of plumb because you capped the metal upturn flange of the tub with the cement board.
Tiling out of plumb walls.
Do you stack the tile.
How do we deal with that doing tile.
Tips for tiling a wall 1.
Hang a batten board to prevent tile slippage.
A batten board helps you start your bottom most course or row of tiles.
So you ve got that bottom row of tile.
We are doing a remodel and have discovered the wall around a tub shower is out of plumb by 1 2 over 5 feet.
Check your floor for level and walls.
If this is not the case then forget i brought it up.
Install the wall tile in a pyramid shape.
How to deal with out of plumb walls for tile.
I know smaller tiles will not look good but if tiles were larger like would this help mask the problem.
Find the midline of the wall from ceiling to floor and mark it with a chalk line.