LightningAC:
All I did was take the code made by SparkFun and changed some things. To be honest, I wasn't quite sure what I was doing. But all of the comments were left there by SparkFun.
The code on the site linked to in the video looks quite different from your code.
For instance, the pulseToPWM() code on the linked site does constrain the PWM value to the range -255 to 255. Your code does not. Hence the comment in the Sparkfun code is correct. The comment in your code is wrong.
In the drive() function in the linked site's code, the constrained values are actually used. In your code, they are thrown away.