Count columns based on sum of previous month's row values once a value is hit

I'm trying to count the number of months it took in the past to hit a certain value (count the months from the right to the left).

Example:

Excel.png

I have hardcoded the values in yellow here, with the correct answers. May 2014 would be 6, because December 2013 through May 2014 (trailing 6 months) summed to 54%.

Hope I am explaining this correctly. Thank you!

For future reference, it would help a lot if you could post a sample workbook rather than a picture as it would be time consuming to manually type all those numbers in ourselves to derive a solution.

Anyhoo, in B4, type in

Formula:

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Copy to the right until J4.

You rock! Thank you!! I'll be sure to post a workbook next time, sorry!

No problem. Glad it worked on your spreadsheet.