If you want to express the actual as a percentage of the forecast, the
formula is:
=actual/forecast.
So, 15 is 15/20 = 75% of forecast 20.

If you want to express the percentage as a deviation from forecast then
you need the difference:
=(actual-forecast)/forecast
So, 15 is -5/20 = -25% less than forecast.

Does this help?
Kostis Vezerides