While translating AD&D 2nd Edition into a d20 OGL styled combat was easy, the translation of Non-Weapon Proficiencies is proving to be a bit more difficult. The mechanic itself is easy – the DM assigns a target result, and the player rolls d20 and adds their modifiers. The DM’s target is easy, as it’s laid out very plainly in the d20 ruleset. Additionally, it is easy to assign a +1 for each NWP slot a character has put into a proficiency. The problem comes in applying an ability modifier.
How did rule this end up? Are you still playing 2e?