There’s a few scenario to calculate a percentage of a measure in OLAP CUBE (Analysis Service 2000). A Percentage is common calculation in MDX , and it can be solved by divide a measure to  total value (Grand Total).

Sample 1:Measures.[Value Contribution to Total]:

1

(Measures.[Value] / (Measures.[Value], Time.[ALL Time], Products.[ALL Products],…)

Sample 2:Measures.[Value Contribution to Total]:

1

(Measures.[Value] / (Measures.[Value], Products.[ALL Products],Warehouses.[ALL Warehouses])

Sample 3:Measures.[Product Contribution to Total Products]:

1

(Measures.[Value] , Products.CurrentMember) / (Measures.[Value], Products.[ALL Products])

Source:

MSDN