If I try to use an item completely outside the toon's class, then UMD determines the chance of success. But what if I have some levels in the appropriate class? Or dilettante in another appropriate class, or both?

I have a level 17 half-elf whom I want to completely remake (via LR +20) into sorcerer. I want to be able to do scroll healing. How would 2 or 3 levels of FvS affect her scroll UMD? Or paladin dilettante, or both? (Cannot splash paladin because she is chaotic good)