You're making this overly complicated.

No matter what, a compact frame's standover height will be LOWER than a standard frame of the same size. So you cannot lose compared to a classic frame and the only question is how much you can win.

In any case, it frees people from worrying over standover height unless they have specific issues like problem hip joints that limit what they can step over (not stand over). It also means that we are free from trying to hit a narrow target between the limits of short seat posts and standover clearance.

The long posts, combined with sloping top tubes means that we can focus on the actual issues of fit, including head tube height and saddle to stem length, which is measured in the classic way - parallel the the ground from seat tube (or post) to the steering axis whether it's the head tube or stem.

As far as standover height goes, I'd take it at the center (or so) of the top tube, which is about where one is likely to stand over it. More info might be useful, such as the height of the tube a the seat tube (frame size in many, but not all cases) and head tube height, but standover height is unnecessary unless one is in that minority for whom it would be an issue.