Hi
Can i know what chip are you planning to use for the PWM? If you are using any standard PWM chips like SG3524 or 3525 the best method is to vary the voltage at pin 9. Connect a 1N4148 diode from pin 9 to the wiper of a 50k pot. The upper end of the pot masy be connected to pin 16,that is the 5v Ref. The bottom may be connected to ground pin. Varing voltage at pin 9 will vary the pulse width to your required value. it is always suggested to get ref from pin 16 as it is highely stable.
thanks
Hari