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.
A batten board helps you start your bottom most course or row of tiles.
You will lay out your first tile directly on this line and your.
Tips for tiling a wall 1.
Measure the wall where you will place the tiles.
So you ve got that bottom row of tile.
If this is not the case then forget i brought it up.
Do you stack the 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.
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.
How do we deal with that doing tile.
Find the midline of the wall from ceiling to floor and mark it with a chalk line.