I just figured here on paper , the way I always do.
Go check out my cabin on the new thread and you can escape for awhile!! LOL I would do a practice one and then you can tell if you are off any and by how much, don't let this get you so wired up. I think you can easily concur it by a little trial and error! Start with a 15 1'2 and go from there. and "BREATH!!!"