Avg for dynamic range

[#1]

I am looking to calculate an average of monthly 'Sales Volume' between the max(month) and a defined date (VSE End Date). If the VSE End Date is more than 12 months ago i want to cap the monthly average goinf back to just 12 months.

=

avg({<Month={'>=$(=max(Month)-column(8))<=$(=max(Month))'}>}[SALES VOLUME])

**Column(8) is where i calcualted the number of months since VSE End Date.

So for example I have a customer who has a VSE End Date of from December 2014 and to day is February 2015, so i would want an average of sales volume for the past 3 months (Dec, Jan, and Feb)

Any help would be greatly appreaciated.

[MK_QSL]

You mean to say Column(8) is the result of some expression, which could have different result on each line.

Please clarify !

[#3]

Yes, column(8) is the result of an expression that will be different for each customer.

[MK_QSL]

If possible provide sample data or apps..

What is the format of Month and Date fields?