@ZerothWell, if you want my humble opinion -
Yeeea, having something 'increase the chances of success' is a little odd - especially in something like alchemy or chemistry. As you said, it either works or it doesn't. If you know something interacts with something in a certain way, then you just have to figure out the right amount of whatever substance gives you the desired result. It either works or it doesn't - there's not much of an in between. As far as I am aware, I've been out of chemistry for awhile now so I could be terribly wrong, and not really a narrative problem but eh.
(Also, just a note - I think I'ma change the end of that collab post a bit, especially depending on the outcome of this little discussion since if Ash knew what to do with her skills, she'd likely keep both the flesh and herb and see what she could do with them. A tiny bit of an oversight on my part, really.)
Increasing the potency of them would be a simple solution, though I don't think the skill itself would have much room to grow if that was all it did. Also it'd perhaps run into an issue where the skill would cap off at a certain level or just get to the point where it was ridiculously OP if she made something like a firebomb or something. Easy mini nuke in the making, but that is me assuming what exactly can be made with alchemy. (Ever play the Atelier series? That's what I'm roughly basing my understanding on at the moment). Perhaps it could start off as a simple strengthening ability, but turn into something else later?
Perhaps something that allows her to do otherwise extremely unique things with alchemy instead of the standard potion making? I'm unsure what constitutes exactly as alchemy in this world of yours, so I'm guessing a bit here.
If you wanted it to be in a similar vein of 'increasing the chances of success' it could be something as simple of a skill that lets her know how well and how efficiently a material could work with another? At a low level it'd only be a general idea of perhaps how 'good' or 'bad' a material would react with another. At higher levels it could let her know how potent an ingredient is, how well an ingredient would react with another, and if that particular ingredient had any special or odd properties.
Basically just a type of analysis skill for alchemy. Effectively, 'increasing chances of success' would be written in practice as 'I can tell how well this reacts with something, but now what the effects are until the skill gets stronger/I discover it myself through trial and error.' With the 'chance increasing' bit basically more or less being intuition, a bit of educated guesswork, and previous experiences. sort of. Not sure if I'm explaining it right, but that's the best I got at the moment. The skill itself would get stronger by discovering new items/ingredients and using said item and ingredients in alchemy. A helpful tool, but one that only grows as the user explores their alchemical abilities.
Okay, went on a bit of a rant there but there's a few ideas from me xD