I don't think there's a standard for reporting frame sizes. Back in the day I'm not even sure if Schwinn did it the same model to model.
Tape measures will get you close enough. Keep in mind if the bike has a suspension, a lot of these are simply baseline numbers. Add springs and frame geometry becomes dynamic rather than static. They'll change as soon as you sit on the bike and the suspension sags, not to mention as it's being ridden and the suspension does it's thing.
You could be right Desmo, I always assume C-T, usually close to reported frame size.
I'm a semi-retired carpenter and am used to working in 64th's of an inch but for my purposes I eyeball most dimensions on a bicycle. (I wish we would all just agree on the metric system, porra!).
Good points about suspension bicycles. I cheat a little and use floor-to-axle on some dimensions (BB drop), obiviously this involves tire size and air pressure at that moment. I started out using a Stabila level to get accurate reach and stack dimensions, for example. Decided it's not worth the hassle and this info is for my own purposes anyway.
I'm still new at this, if I mess up let me know.