When you say "MOD 32 ripple counter", do you really mean just a ordinary 5 bit counter?
Your counter has 32 discrete states (if I understand what you mean). Note that 5 (1 / 20%) doesn't evenly divide into 32. If you're really stuck with the 32 states, then the best you can do is 6/32 = 18.8% or 7/32 = 21.9%.
If this counter has a reset input, you can set it up to go back to 0 after 4. It would count 0,1,2,3,4,0,... which is 5 states. By detecting any one of the states you get something asserted 1/5 of the time or 20%.
In general, PWM uses a counter with comparators for two different states. One resets the counter and therefore defines the PWM period. The other causes the output to flip to the off state, which was initialized to the on state by the reset. This value divided by the first defines the duty cycle fraction.
