With that size tile the best thing to do is remove lath and plaster and board with plasterboard. Fix the tiles directly to the board DO NOT SKIM fix the board with screws not nails. gypsum plasterboard can take 32kg/m2.
The customer admittedly dos'nt want that mess of lath and plaster being ripped off and I have over boarded with 12.5mm plasterboard onto and fixed to the studwork. Again the customer was wrong and should have asked your advise before buying, But they don't:huh2:
i am currrntly doing 600mm x 300mm and these are 10mm thick i am using an 2mm space depending on the effect off the tile really i would say i.e if marble use like virtually no space