Thanks for pointing this out; you're right to question it, as the success chance was actually 10%, not 25% as stated in the game. I've fixed this now. If you're successful once with it for a specific girl, then the chance doubles to 50% for her thereafter.
No problem! If the girl refuses the concentrate, you suffer a hit to affection, but the success chance isn't changed as it's fixed and unaffected by anything else (as you identified).
As the concentrate is so powerful, I wanted to ensure it retains a low success chance, regardless of any upgrades or affection modifiers. It's tricky, as I don't want it to replace the core game feeding mechanics, but at the same time, I don't want it to be too frustrating to use. I may need to tweak it further in the future.