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.