Report - Pull Percentages for Each Group (Same Report)

I'm trying to create a report that displays AUM by lead source across four different years.  The report is grouped first by year, and then by lead source within each year.  I want to calculate the percentage contribution of each lead source to the total AUM for that specific year.  In other words, the percentage should be based on the subtotal for each individual year — not the grand total across all years.   

Each year’s total should represent 100%, and the lead sources should show what portion of that they account for. 

I'm trying to use a formula field to calculate this, but I’m encountering the following error: 

“Incorrect number of parameters for function 'PARENTGROUPVAL()'. Expected 2, received 3.” 

Here is the formula I attempted to use in the report: 

RowCount/PARENTGROUPVAL(RowCount, Account.Funding_Completed_Date__c, COLUMN_GRAND_SUMMARY) 

Assistance is very much appreciated!  I also included a screenshot. 

Thanks, 

Michael