Calculation percent difference

Hi i am trying to do the following.

Calculate the percent diff between to columns

So i want to take the Waitlist sum this then sum the overboundary and divide this to get a percentage, as a line total not just a grand total

 any help would be appreciated

Overboundary calculation =

Works

=Count(IIF(Fields!OverBound_Flag.Value="Yes",0,Nothing))

What i have created, not working in the % column

=Sum(Fields!WaitingListId.Value)/Sum((Count(IIF(Fields!OverBound_Flag.Value="Yes",0,Nothing))),"OverBound_Flag")

David