Crystal Reports Opposite of Running Total?

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     I can't think of a better way to put it, and that's probably why my online and text searches aren't returning any results.

Essentially, I have one currency field in a group header that I need to draw down on for each record in my Details section, with the balance carrying forward so that each record subtracts not only its own value, but includes all previous records' values, as well.  So, the opposite of a running total, in that, instead of the number gradually increasing, it gradually decreases.  The best example I can think of is your bank statement, where each transaction deducts itself from the balance of your account as of the previous transaction.

I know there's a super easy way to do this, but my brain isn't functioning well enough to remember it, and I would greatly appreciate any help anyone out there could offer.

Thanks in advance!