Looking for a formula or formulas that will help me figure out how much more each month that I need to sell to make up for the previous months down (or up) sales based on a stated 20% annual growth goal.

I want the rolling future sales goal to adapt to the previous monthly sales that I input as the year progresses. I.E. I know the size of the pie I want, and I know that I have 12 slices and as I go through the year some of those slices are small and some are large, but what I don't know is how big the future slices need to be to complete the full pie.

Hopefully this makes sense. This is my first post on the forum!