Just take out a calculator and lay it in front of you and everytime you stop the action, then add it in into the calculation to see which ones need to be tweaked.

Or, eg if you need 70 Conjuring, and you have to use 3 ingredients, then get all three ingredient to about 70 Conjuring each, then you don't needto calculate.

I find that eg, if she want 70 +/- 2 Conjuring, stopping it at 68 or 72 and throwing it into the couldron deosn't work, maybe its just me...