I discovered this trick when i was calculating how much pips I needed for tempest do enough damage when it had a boost. Here's what you do: Take the number or damage you want, in this case 85, and then multiply it 0.30, 30 being the percentage of the boost. This is especially helpful when using X rank spells, to figure out how much pips you need, but in order to do that, you need to multiply by a certain number, so play a bit of hot and cold between 1 and 7.
Please tell me if you think this is helpful for you, because I would appreciate it.