I wonder how SE expects us to test this. Not easily apparently. I can think of a few ways to get the desired skill levels but they're time consuming and won't get a good effort/reward ratio for information unfortunately (well, unless that bug where setting a job to lower levels also lowers skills on other jobs is still around).
Since I have doubts that SE is going to allow us to edit skill levels for this I'll have to give up on it for now.
If you feel up to it can you test two things? I'm not sure they're worth testing but I might as well ask in case you think they are.
- Does Healing Magic Skill effect numbers under a brew?
Naked, capped Healing Magic, 99 WHM, Primeval Brew for Cure I-VI.
A higher amount of Healing Magic (as high as possible using whatever non-potency gear you have and merits), 99 WHM, Primeval Brew for Cure I-VI.- Is the new effect of Healing Magic Skill just multiplied on top of cures?
Cure IV on Scholar without Light Arts, with and without the cure potency staff.
Cure IV on White Mage, with and without the cure potency staff.
Honestly I'm thinking Motenten probably has a better lead onto this then I do but I'm not going to give up on trying to figure this out in a creative manner yet.