Hi,

Caveat, I've never used or done this before but. . .

there is a Matrix.Columns Property available in SSRS, which will return the number of columns in the report. Based on that, you should be able to determine how many months are being used, and thus calculate an average.

I believe that you will have to write some custom code to get to that property (Report, Report Properties, Code) or write you own class/assembly using the language of your choice.

Otherwise, I would consider calculating it ahead of time in your SQL if possible.

Good luck.