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.
In the d20 implementation of the 3e ruleset, WotC created a single modifier table for all abilities. An ability score of 10 or 11 was considered to be perfectly average, with no bonus or minus. For each pair of points about that, the character received a +1 to their Skill Check (the 3e version of an NWP). So, a character with a 12 or 13 receiving a +1 to the appropriate skill check, and a character with an 18 or 19 received a +4.
For comparison sake, below is a table of the d20 OGL 3e ability score bonus and the various 2e AD&D bonuses: