Hari, a possibly completely different solution, depending on how you're consuming the data. We use Pivot Tables for a lot of our analysis, and you can just right click on the measure in the pivot table, choose Field Settings, and use % of Column. Voila, you have % of the diagnosis.
If that doesn't work for you, create a new calculated member like this:
([Diagnosis].CurrentMember, [Measures].[Claims Count])/([Diagnosis].CurrentMember.Parent, [Measures].[Claims Count])
Then switch the format to "Percent" just as omlac suggested.
Hope that helps,