[Tristan Lall]

In general, the CIM motors appear to be relatively balanced (forward vs. backward), but there will inevitably be some bias, due to manufacturing tolerance (contrast this with some motors, like the old drills, which were apparently biased deliberately). There is no universal number which will allow you to account for this tolerance; it's a matter of scrutinizing the motor's performance. Fortunately, the effect should be essentially negligible, especially considering the fact that the two sides of your drivetrain (assuming that that's the application) will have efficiency losses which won't necessarily be evenly balanced either.