I would use a linked table to assist with the calculations. Make a table called [Contractual Years]
. The primary field would be a formula to concatenate two other fields where you specify the start and end month-year combo for a given contractual year:
In your primary table, you would link to a specific contractual year for each month. On the [Contractual Years]
table, you would add a rollup field that would tally all of the monthly revenue values for linked months.
Back in your main table, you would add a lookup field to pull in this total.
A {Percent of Total}
formula field would do the math to determine how much each month factored into the total.
IF({Total for Contractual Period}, {Monthly Revenue} / {Total for Contractual Period})