Piston rings and size . Help

[Talkwrench]

Guys I need help, building a 37. 221 and the pistons I have ( ford 3 ring domes ) are stamped . 030. I purchased .030 over rings and went to fit and there was a .030 gap..? I ordered another set thinking they must have been in the wrong package, well these are also .030 gap. The machinst said the pistons must be marked incorrectly and to get .040 over rings. Yes the pistons fit nice in the bore. So I'm just double checking that this sounds right, is there a formula to use knowing that there is a .030 gap now??

Cheers.