Here's what I did in a similar situation, if I understand the original question.

One, of course, cannot directly CALCULATE backwards. However should the table's order be controled by an index tag, one can SET ORDER TO TAG DESCENDING or ASCENEING and then employ the CALCULATE or SUM commands to "work backwards". Both commands support scope (all/rest/next n) as well as FOR and WHILE expressions that should permit selection of the desired records.

Just another way.