On these Ross pistons the clearance is .005. It really depends on the material of the forged piston and the relation of where the oil hole is to the oil ring groove. That design determines how cool it runs and how much expansion will take place with the nitrous.
I give the specs to the piston manufacturer and tell them the intended use. They do the calculations from there. But I've gone as tight as .002 and as loose as .007.
