My head hurts even thinking about the formula lol, maybe use c02 to get a rough idea? I think at 80f c02 is 850 psi.I'm sure you can find the fps of a 13xx and a 2240 with the same barrel lengths.at least you could figure out if your in the ball park.
I suppose for serious testing or r@d you could drill and tap a pcp pressure gauge into a "test" gun, but that would change the cc of the valve slightly.I'm always interested in this kind of info, the more you know the more you can do.