10mm square notch for me and i like floor stuff runnier than tubbed gear but not pourable. You should be able to push the tile into it yet it should keep its notch when spread(as grumpy posted).
Worked with another tiler a few weeks back on a big floor he liked it real thick, pain to spread and couldnt push the 600x600 into it so we mixed our own and i was doing double his m2