The first answer I'll give you isn't an answer at all. The formula's a waste. Doesn't work. It's wrong. I've been working on a new one, but having gotten this close and failed, it's a little difficult.
The second is that whatever answer you get in the box marked 'weight 1 location' and 'weight 2 location,' divide them by 2.54 to get the number of inches from the butt cap they are. In they're current form, they're the same number, but in cm.
Diablo Tour 93, TNT2 16 @ 56#, Prince DuraRib+
And, returning after more than a year under a rock... Amone!