good afternoon,
this website says a 12x12 square notch gives you 6mm deep adhesive. it also says a round trowel leave a 1/3 of its size, so a 10mm round lays 3mm deep.
https://www.rubi.com/en/blog/how-to-choose-the-right-tile-trowel-size-step-by-step/
my tiles are 400mm x 150mm, 10mm thick...