your on the right track, take 1 case, punch it out to 30 cal with an expander ball, the neck size till the action closes with a bit of resistance, then screw your die down 1/16th turn. resize again and measure that case compared to a factory 7mm cartridge, if they are the same you just load as normal within the pressures you need to use. if they are different you may have to make all your cases for it, not a big deal once you know what size to make them.
RR
edited to add, or you could order go/no go gauges for a 7x57