[Bob Koehl]

Once you are convinced the math is OK, there is still one more possible thing to check.
The Limit Mix function absolutely requires that the joysticks be calibrated.
If the calibration is off, the robot will go in different directions than where the joystick is pointed.
The lights on the operator interface show if p1_y and p2_y are calibrated.
For single joystick, the p1_y and p1_x need to be calibrated.
Calibration is when both y and x values are 127 when the joystick is centered.
Hope this helps.