At the core, this is a simple formula that simply divides the total amount by the number of months given:
The trick is to "cancel out" this amount in months where it doesn't apply.
To do this, we use this logical expression:
Here we use the AND function to test each month in row 4 to see if it's both greater than or equal to the given start month, and less than the end month, calculated by adding the start month to total months.
AND will return TRUE only when both conditions are TRUE, and return FALSE in another other case. This effectively zeros out calculations in months that fall outside the range of interest. This works because during math operations, FALSE is coerced to zero, and TRUE is coerced to 1.
Without named ranges
The formula in the example shown uses three named ranges. Without these named ranges, the formula can be written like this: