Adding flexibility to a forecast

I am trying to create a calculation that if I change a date, it'll update the forecast. It's hard for me to describe so I have attached an example.

Basically I have 12 months that need to be populated with budget amounts. Depending on the month the spud date occurs, is when I want the calculation in the forecast (shaded light blue) to begin. The only input I want to change is the spud date (shaded in yellow). Currenlty I have forced the months to match on Wells 1 & 2, but I want to have the flexibility to change the spud date and subsequent equations would be updated.