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], [...]