Yes, tourney limits are just that, limits. 12.5 is the upper limit of what you are allowed to do. In your case, just set it to 12.0 bps.
With the APE aftermarket board, you can set bps in .1 increments, so if you wanted you could set it to 12.1, 12.2, up to 12.5. But the rules are just that you cannot go over 12.5.
I just got a G3 Spec R (very similar to the G4) and I LOVE it. Great tourney gun so you will do just fine.