I don’t add any, but I’m looking at some notes I received from a gentleman who runs a successful shop that does use a semolina infused dough and he uses about 10% and said that up to 18% is good.
I use 11% in my regular dough recipe. I have heard going higher near 20% is not the best idea. Frankly when I don’t use it I feel the final pizza is missing a little flavor and crunch.