Get a good flat piece of 3x2 or 4x2 to tap the boards flat. Make sure the corner by the bath is a perfect 90 degrees. Un-skimmed PB will carry about 30kg per metre off the top of my head but that includes adhesive.
I've watched that video a dozen times and I still don't know how to tile!! I'll try again after my cold shower.
Utterly fantastic, although I didn't learn much about ceramic tiling, I did learn I'm a pervert.......