Query scanning all partitions in result set

I have a scenario where I am returning 52 weeks of data, but for one calculation I only want to return the latest week. I am getting poor performance because it is scanning all partitions in the 52 week result set. I would like it to only scan the last partition if possible.

I tried writing something like 

MEMBER [Measures].[Number Locations] AS 
( 
		iif([Date Dimension].[Fiscal Year].CURRENTMEMBER.MemberValue = [FiscalLastDay].Item(0).Parent.Lag(1).Item(0).MemberValue ,
		[Measures].[Locations Sold]
		,Null)
)

But it didn't seem to help.

Locations sold is a calculated member

SCOPE([Measures].[Locations Sold]);
    THIS = IIF(ISEMPTY(measures.units),NULL,
            COUNT(NONEMPTY([Locations Sold].[Locations Sold].[Location Dimension ID].members,measures.units)));
END SCOPE;

FiscalLastDay is calculated as follows

CREATE HIDDEN TodayDate = Format(Now(), "yyyyMMdd");
CREATE HIDDEN SET [FiscalToday] AS 
    Filter([Date Dimension].[Fiscal Year].[Day Name].Members, [Date Dimension].[Fiscal Year].MemberValue = StrToValue(TodayDate));
CREATE HIDDEN SET [FiscalLastDay] AS
    [FiscalToday].Item(0).Lag(1);