+1
This is the important point. Forks settle when you sit on the bike and the dimension you want to match is the settled one.

An easy way to measure is to put a cable tie or twisted pipe cleaner on a stanchion, loose enough to slide, but tight enough to stay put on it's own. Sit on the bike and the slider will be pushed up. Now measure the gap between loaded and unloaded position on the stanchion, and subtract that from the unloaded axle to crown distance. That will give you the target dimension to match.

IMO, anywhere within an inch or so to either side of the target is good enough unless you're an expert bike handler who might feel he difference. Depending on how much drop or raise you end up with you might want to compensate the stem height, but that can wait until you ride it a while. Who knows, you might actually prefer the new height.