How do I move one value to the next month?

I have a calculation currently in the "Usage Base Forecast" line item. I want it so that if a user checks off the "Use Previous Month Final Forecast", it moves the final forecast from last month to next month. For example, if the boolean is checked off for July, it will take the "Usage Final Base Forecast" and move it to August. I'm thinking that I need a POST or OFFSET formula?


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