600 x 300 ceramic wall tiles.

[heavytrevy]

Id go back to using a 10mm with a back skim and use a rubber mallett to tap the tiles flat, hope that helps mate.

Trev

 

[Wylie]

is it not a matter of taking a bit off,putting a bit on and sticking with the 10mm,check the walls with the plumb aswell,you know what way its running as a twist in the wall will add to your problem

 

[TheFunkyMonkey]

A Customer turned up with 600 x 300 ceramic floor tiles on a job I did earlier this year. Changed their mind at the last minute. I ummed and ah'd a bit, and got on with it.
Was expecting 400 x 250 porcalain in regular pattern, so didn't pay too much attention to the flatness of the walls. 3 rows in, I was really struggling to get them all plumb in the stretcher pattern. The walls were all a bit bowed and twisted due to the studwork.
Bathroom i'm doing at the moment is a similar house, with 600 x 300 porc in vertical stretcher pattern. boarded it out today and got a long straight edge all over it in all directions. It's important to check all directions, not just horizontal and vertical. Any off bits, I level them out with addy before starting tiling. As said above, it needs to be almost perfect or you'll be pulling your hair out as to why they won't line up!
Big tiles and stretcher pattern = hard work!