Running Total (MDX)

WITH
member Time.CurrentDa as ([Time.Weekly].CurrentMember.Level.Name = "(All)")
member Measures.Now as 
IIF ([Time.Weekly].CurrentMember.Level.Name = "(All)",
    IIF ([Time].CurrentMember.Level.Name = "(All)",
        0
        , [Time].CurrentMember)
    ,[Time.Weekly].CurrentMember
)
member Measures.Prev as 
IIF ([Time.Weekly].CurrentMember.Level.Name = "(All)",
    IIF ([Time].CurrentMember.Level.Name = "(All)",
        0
        , [Time].CurrentMember.PrevMember)
    ,[Time.Weekly].CurrentMember.PrevMember
)
member Measures.Gain as 
CASE WHEN Measures.Prev > 0 THEN ( Measures.Now - Measures.Prev) / Measures.Prev
    ELSE null 
END, FORMAT_STRING = '#0%'

SELECT
NON EMPTY {[Measures].[Unit Sales], Measures.Gain} ON COLUMNS,
NON EMPTY {[Time.Weekly].[Week].Members} ON ROWS
FROM [Sales]

--Screenshot: http://cl.ly/image/1M2k2Y1E0c1X