Hmm... wondering where my post went... Turns out I did the thing wrong. 😀

The way I got it to work was to create two datasets - one of the standard "referrals" dataset that gets filtered by the @ReferralType parameter, and another that contains as few fields as possible and does not get filtered. (There's no sense in returning a lot of extra data - I only want the count of members.)

Here (hopefully) is the report that actually works. In the unfiltered dataset, I'm just returning a single column - basically, enough to get a count back. I'm using it to assign a value to the variable @PopulationSize, which I can then use to divide the count of the filtered recordset by.

Okay, okay... here is the fixed report. Sorry about that!

Here's the calculation for the variable, PopulationSize:

=CountRows("UnfilteredReferrals_dset")

Then in the report, the expression for the percent of Population Size is:

=COUNT(Fields!PersonID.Value)/Variables!PopulationSize.Value