Grumpy_Mike:
It's actually called "calibration"
No
No this is not really calibration, fiddle factor is more accurate. However, you can try and calibrate your fiddle factor.
However, yeah, if you do what mike said, you'll be able to at least get the thing to drive in a straight line. I had to do something similar for a microcontroller class and our solution was to find PWM factors for both motors such that they ran at the same speed, and then work from there. It wasn't the best, but it didn't involve extra hardware either.