Ask HN: Advice on factoring percentages for PWM SSR power driver

I could use some help developing a function for controlling the on off timing of a Solid State Relay which is driving the heating element in an electric still.

The SSR will only switch while the AC wave is at the zero crossing, thus limiting the minimum switching time to approximately 8.3 ms. For stability I would like to set a minimum of 20 ms switching time.

I would also like to set the power level between 0% and 100% in 1% increments. My current solution is using the formula "%Power X 20 = timeOn / 2000 - timeOn = timeOff"

Although this works, unfortunately it results in longer than necessary on/off durations in order to get the intended duty cycle.

Eg. A 50% power level would be 1000ms On / 1000ms Off. This would not be as efficient and harder on the element as it would be to get 50% power by having the element 20ms On / 20ms Off.

Hopefully I have explained my problem in such a way that makes sense. What I believe I need to do is factor the larger fractions created by multiplying the the percentage, down to as small as possible with a "numerator On" or "denominator Off" value of no less than 20ms.

Any thoughts or advice or articles/tutorials covering this subject would be appreciated.

Thanks