Calculated measure in last X days

Hi all,



I'm trying to create measure that will aggregate calculated measure over last x days (can be any number).

On the axis 1 I have the dates . I can't use the sum function cause it's a percent measure. I also cannot do sum separately for the numerator and denominator because I need to implement this measure over complex measures that I don't always know how they are constructed.



I tried to do the following measures:


with 
member [Measures].[avg sale] as
[Measures].[Internet Sales Amount]/[Measures].[Internet Order Quantity]
member [Date].[Calendar].[last 7 days] as
AGGREGATE(LASTPERIODS (7,[Date].[Calendar].CURRENTMEMBER))
MEMBER [Measures].[avg sale over last 7 days] as
([avg sale],[Date].[Calendar].[last 7 days])

select {[avg sale],[Measures].[avg sale over last 7 days]} on 0,
    [

Date].[Calendar].[Date] on 1
from [Adventure Works]


I keep getting null in the result, it's seems like the aggregate cannot evaluate the currentMember function.



does anyone have an idea how to overcome this?



Thanks,

Liran