There probably is a rule of thumb measurement somewhere if you google for it, but personally I wouldn't bother.
Shoulder your existing rifle / stock - is it too long, too short, just right? and base your measurement on that.
If you get the measurement wrong, it's not the end of the world, if it's too short - fit a thicker butt pad or fit a spacer. If it's too long you might need to cut a bit of stock off. Fitting a spacer/different butt pad is less traumatic than cutting your new stock so I'd say it's better to be too short than too long.
13½-14 inches is a good starting point I'd say.