[Runtimemicro] has the answer, at least if you are writing PWM code for the Arduino. Their free application lets you set a few parameters, visually see the results, and then generates code for you.

What Are PWM Pins? The Arduino Mega's PWM pins are digital output pins located on D2 to D13 and D44 to D46, coming to a total of 15.

The machine uses a ‘grbl’ controller, which has an Arduino-based circuit that includes multiple outputs, including a powerful 24V pwm signal that is generally used to drive a 60-80W spindle motor.