WITH MEMBER [Measures].[Percent of 
  Total] AS ' ([Store Sales],
  [Product].CURRENTMEMBER) / ([Store 
  Sales], [Product].[All Products])',
  FORMAT_STRING = '0.00%'
SELECT {[Measures].[Store Sales], 
  [Percent of Total]} ON COLUMNS,
  [Product].[Product Family].MEMBERS 
  ON ROWS
FROM Sales