CF Report Builder percent of total for group

I have a report (built with CF Report Builder) that calculates subtotals in groups. My user wants to also see what percentage of the grand total these group totals represent. It seems that CF ReportBuilder doesn't know what the grand total is until it gets to the end of the report. The percents it's showing are the percent that the current groups subtotal is of the current total. For example, the first groups total is 19.5. The percent of the total is saying 100, because that's the first group and the current total is 19.5. The second groups total is 148.4. The report is saying that percent of the total is 88.3 because 148.4 is 88% of 148.4 + 19.5. The actual Grand Total of the report is 347.48, so the second group should be about 42.7%. How do I get this calculation to use the Grand Total of all the groups, including the ones it hasn't printed yet?

Thanks,

John