Well, for instance, say you're adding 12 grams total. You might enter as mass 1 anywhere from '1' to '11' and that's how much weight you would add at the location specified for mass 1. It's really just a reference number, like that most classic of variables, x.
The choice of how much weight to add at location 1 or 2 is purely your choice. In the creation of this, I made a simpler calculator, using set masses, then added in variability afterwards.
Diablo Tour 93, TNT2 16 @ 56#, Prince DuraRib+
And, returning after more than a year under a rock... Amone!