The instructions that came with my Ross forged pistons are:
Normally aspirated:
Bore 3.475 and under .003
Bore 3.476 - 4.499 .004
bore 4.500 and up .006
Marine +.002
Nitrous applications over 250 HP +.002
It also says filled blocks and blocks honed without plates may require more clearance.
My .030 over 390 was done at .006"
Bob
tufcj