If x then y for a Custom Character Sheet for Table Top RPG Game

Hello all,

I am in the process of creating a Custom Character sheet as a PDF form using Acrobat Pro X but I need a Javascript that will help me fill in some cells properly.

I want a modifier (Ability Mod) that shows to be generated from the Ability Score. When I say that, what I mean is the characters get a bonus (or negative in some cases) to things they do depending on their Ability Score. Like, if a Character has a score of 15 for their Strength, they get a bonus of 2 for things they will do that use Strength. So, I am trying to figure out how to make it so that when the Ability Score is determined, it will AUTOMATICALLY put in the Ability Modifier into it's respective spot depending on what the Ability Score is. 

I have attached an image in hope that it helps explain what I need better.