The limit mix function is a bit of software that takes the two axes from one joystick, and calculates the corresponding PWM outputs. In order to adjust a PWM value by a constant factor, try something like this:
Code:
pwm01 = (pwm01-127)*0.75+127
There may be a typecasting issue there, so you'll need to play around with it. However, the idea is that you need to recenter the value around 0 (by subtracting 127), lessen the magnitude (by multiplying by less than 1), and then shift back to the normal range (by adding the 127 back). In this case, this code would reduce a motor to 75% of its original speed. If that doesn't make sense, feel free to ask for more clarification.