Two things:
1. I also have mulitple pay codes: Preload, Air Driver, Cover Driver. I use an app called HoursTracker. Once I've entered my pay scales, put in my day, and add my break - I am spot on. It got bad at my center until I started pulling out my phone and showing pay discrepancies. Now I take pride in showing them how often I am "to the penny" with my check.
2. Working preload then driving is a rip off if you don't know how they screw you. Here's the catch - If you are scheduled for preload and then they need you to drive you will get preload pay THEN driver pay. If you are told "we need you to drive tomorrow but can you come in and help out on preload" or if you are scheduled to drive and get called in to help THEN and only then do you get driver pay for the whole shift.