How to size a corset?

I get it. The usual advice for a first corset is to take your natural waist measurement and subtract four inches. But what counts as “natural”? Are you breathing in or out? Are your abs flexed or is your belly relaxed? That range could easily be anywhere from 29” to 31”. I know my trouser size, but that was determined by trying on pants—which usually sit lower than the actual waist—so if those numbers matched my true waist measurement, it might just have been a happy accident. I asked because my eBay corset (which closes at 28”) arrived yesterday and I realized I can lace it all the way shut already:-( It feels lovely, but I want tighter. Maybe I should have sucked in my gut when measuring around my navel. I’m planning to swap it out; I just haven’t decided if dropping down by 2” or 4” is better. Since I’ll be wearing it occasionally rather than for structural reshaping, the 2” reduction seems more logical, particularly since I expect to wear it for periods longer than ten minutes…