The American Tile Council doesn't specify any specify saving for tile. Some tiles do have recommendations from manufacturers. In general, you want to consider the consistency of the tile size as well as how even the substrate will be. The more inconsistent the tile size, the larger the spacing....i.e if you tri to do an 1/16" grout joint on a tile that varies 1/8" in size, you will end up with spacing issues. The substrate being less than ideal makes lippage (small differences in tile height of adjacent tiles) a bigger concern as well not to mention the skill of the tile setter. I know that's not a direct answer, but should give you some guidance.

Hi there. Here is a great article and video on Cement Tile installation. Here.

https://www.prettyhandygirl.com/how-to-install-cement-tiles/