I've been advised on here I need a 10mm half moon adhesive trowel to lay my 610mm x 405mm 12mm thick travertine on the plasterboard walls and marine ply floor but I'm a little confused how the size is measured is it the depth of the notch or the width of the that relates to the size. If its the width it will mean a 10mm halfmoons notches are only 5mm deep which I'm not sure will be ok for my floor as its a bit uneven.
don’t worry about the dimensions - just go and ask for / buy a 'thick bed solid bed trowel' it will have the right half moons for you. Buy a Rubi or BAL trowel, defo not Genesis (they are sharp, and very poor quality, even the spot welds suck).
The thick bed solid bed will deliver a min of 3mm solid bed of addy to your floor. The equivalent is a 10mm notch and back butter, but just use the right trowel, it will cost you less than £15 and guarantee 100% coverage (which you need to support soft trav especially on wood or with ufh) if you use it correctly.
As its trav and as its floor you need to fill / skim the cavities in the back of each
tile before you put it down, or the filler / gap will burst through when you get floor traffic.
You are largish format so put a straight edge over your floor, it has to be spot on or you will get lippage. (even make sure all your screws are subflush!)