calculated measure for a varying time period

[lanceg]

Hello! Given three dimensions in my cube (Product, Calendar, & Sales). I'm trying to create a calculated measure that will sum 8 weeks of sales starting from the third week in which sales started. For instance, weeks 1-4 had zero sales. Week 5 is the first appearance of any sales. Therefore, the measure would sum the sales of weeks 7-14 (8 weeks starting with the 3rd week after sales begin).

Any ideas?? Keep in mind that every product is going to be different (sales start in different weeks).

Thanks in advance for ANY help! Lance

[lanceg]

Any ideas?