It's long to display so much data so I've attached a workbook sample. Basically you can just do some simple sums. However, it's tricky with the first month, unless you have 12 months of dummy data (zero value) ahead of it. Otherwise, you need to seed the first 12 months, or otherwise use some more sophisticated formulas to determine how far back to go. With dummy data for the first year, its just a simple formula to always bring in the prior 11 months of revenue. Keep the same sheet across multiple years - so many people try to make a "new tab" every month or every year and its just so much more work that way. See sheet 1 in the sample workbook.

However, I would prefer a larger grid that you put the actual monthly amount to recognize into each month's bucket. This will allow you to change your defferal allocation percent at will, without any loss of prior month's allocations or need to create any new formulas. It also provides a semi-graphical display of where the $$ are coming from each month, which is useful for auditing (less chance of errors and easier to see what is happening, plus more opportunities to analyze the data - sum down, sum across, or sum different areas of the grid as needed). See Sheet 2 in the sample workbook.

<a href="http://northernocean.net/etc/mrexcel/20140115_DefferredRevenue.zip">SAMPLE WORKBOOK</a>

sha256 checksum (zip file): 77c7df6cc5eac665f98e3163ba27b58b3a84b018708cd6694067152ac3e30c13