This question intrigued me so I did a little experiment and got it to work.
Let's say that the field from which you want to calculate your reverse order running total is called "Value".
Define a summary field, sTotal as the Total of Value.
Define a second summary field, sRunningTotal as the Total of Value, but specify that it be a running total.
Define a calculation field, cReverseRunningTotal as:
sTotal - sRunningTotal + Value
And this calculation field will produce your reverse order running total.
This worked wonderfully!
Thanks for being there,