In order to measure stack, you need a horizontal line. That horizontal line passes through the wheel axles, so you need to know something about the fork. In fact, if you have a frame with horizontal rear dropouts, exactly where you put the rear wheel in the dropout can affect the horizontal line so can affect stack and reach (obviously, not very much).

The guy who devised stack and reach verified that if I handed him just a naked frame he wouldn't be able to measure S&R from it.