TOM build platform accuracy adjustment

I purchased my TOM in order to build 3D objects, of course. I've designed most of my objects to max-out the X and Y limits of the build platform (I originally had the ABP, but removed it in lieu of the HBP... because it increases your build height and provides a flatter build surface. I'm interested in build capacity & quality, not printing things as fast as I can).

I measured my build platform, and found that it is 120mm X by 120mm Y... however, I have the HBP with hex bolts at 4 corners a