I think what Gary's saying is - that's very interesting and ingenious - BUT ..... probably doesn't really yield the results you want.WTAF bro? ?
For starters, don't worry about sag of the bike on it's own. Suspension sag's on it's own, some more, some less.
Secondly, sag is kind of a guideline and then how you position yourself on the bike when measuring it for instance - and how you weight yourself, will hugely influence the results you will get.
From what you've written, it kind of sounds like you've decided what pressure you should have and then have manipulated the way of testing your sag to make the sag fit that ???