Replace [Internet Order Count] with base

and [Measures].[Customer Count] with purchases

from the below example.

I have created a [hidden measure] which calculates the purchase/base for every date member.

The [RequiredMeasure] with sum the children which will result in the calculation like you requested.

CREATE MEMBER CURRENTCUBE.[Measures].[HiddenMeasure]

AS null,

VISIBLE = 1 ;

SCOPE([Date].[Calendar].MEMBERS ,[Measures].[HiddenMeasure]);

THIS=iif([Measures].[Internet Order Count]=0,NULL,[Measures].[Customer Count]/[Measures].[Internet Order Count]) ;

END SCOPE;

CREATE MEMBER CURRENTCUBE.[Measures].[RequiredMeasure]

AS null,

VISIBLE = 0 ;

SCOPE([Date].[Calendar].MEMBERS ,[Measures].[RequiredMeasure]);

THIS=sum([Date].[Calendar].currentmember.children,[Measures].[HiddenMeasure] );

END SCOPE;

Hope it helps