Not sure what you mean by "permanent field" here:
"i was able to successfully capture the denominator using a simple Case statement with GetSummary, but decided against that path because i would then have to copy that calc field to a permanent field which would not be continuously updated."
Defined as a calculation field (not a number field with an auto-enter calculation--not sure if that'll update correctly or not) this will be an unstored calculation. (All calculations that refer to a summary field or a field in another table can only be unstored calculation.) It should update continuously and should give you the correct subtotal--provided you have the correct records in your found set and they are correctly sorted with a sort order that includes the getSummary function's "break" field.