I'm using Flextile 52 (white mortar) for both the floors and walls.
I'm setting 18"x18" marble tile.
Everything is flat, level and there is no lippage.
The Following Day
When backerboard is installed by other tradespeople, it's not uncommon for walls to be bowed causing a very uneven surface. It's up to the Tilesetter to correct this as he sets the tile. See below.
There is enough mortar added behind the tiles to bring them into the same plane as all the others.