That formula is pretty close on most "avg. build" people. Get someone w/ really long arms, broader than average shoulders, short arms, etc, and it can be off, but usually it pretty close for the "average" built person. As for the grip thing, you're probably "choke" gripping the bow, and you need to use a more relaxed open hand style grip...look at this article on Gripping the bow...
http://www.bowsite.com/bowsite/featu...grip/index.htm
It has some good helpful tips. I rest the bow on the palm of my hand w/ nearly all of the pressure being on the base of my thumb, then I rest my finger tips gently curled on the front edge of the riser. A wrist-strap helps some people as well, as it allows them to relax a bit more without being afraid they are going to drop their bow.....
Also, I'd be cautious shooting a bow with strings that old, the last thing you want to do it end up with an injury from a hobby. Strings with that much age on them are like time bombs......