[AbsoluteMonkey]

Actually, I think I may have figured it out. It seems to be based on the percentage of a full value. You get 100 percent with a +3, but the percentage decreases as the cauldron decreases.
Here are my values to back up this theory.
Recipe: Balm of First Aid
Cauldron +0: 244 | 33 percent
Cauldron +1: 364 | 50 percent
Cauldron +2: 516 | 73 percent
Cauldron +3 700 | 100 percent

Recipe: Philosophical Phosphorous
Cauldron +0: 403 | 33 percent
Cauldron +1: 610 | 50 percent
Cauldron +2: 881 | 73 percent
Cauldron +3 1202 | 100 percent

These percentages are approximate. As they seems to range +- a point or two. This variation may be based on other stats/levels.