Dimensional Query - Count Cumulative Months Since Fixed Month Member

[LDJB81]

Hi

I can see me banging my head against the wall for the rest of the day so I thought I would post my problem on here. I have a requirement to report cumulative values from the member February 2012 to the current month. The cube is SSAS 2008 and there is calculated item for the current month that I can just drag in - item(current month,0). I was thinking about using a lastperiods function where the integer is the cumulative difference in months, can anybody help here? This is how I see the final expression looking:

Total(Measure with set lastperiods("Diff",item(current month,0))

Thanks
Lloyd