you could always write a scripted process to do the calculations and store them as records in a table.
And here comes da pain.... Now I need to find a way to store the results of the summary fields in the subsummary parts in a table (can even be the same table), since I want to use them as stock out for my stock table (which is a different one). If this was excell, I could simply make a sum of each subsummary part and have the data stored, but in filemaker I am this stuck in the mud I'm about to fossilize....
Since this is FileMaker , you can get the value per sub-summary part using the calculation
GetSummary ( summaryField ; sortFieldOfSubSummaryPart )
Create another summary field sCountAll (= count of primary key), and write a script like this:
# find your records
# sort by product(ID)
Go to Record ( First )
Set Variable [ $countThisGroup ; GetSummary ( sCountAll ; sortField ) ]
Set Variable [ $sumThisGroup ; GetSummary ( summaryField ; sortField ) ]
Set Field [ Table::fieldWhereToStoreTheResult ; $sumThisGroup ]
Exit Loop If [ Get ( RecordNumber ) + $countThisGroup > Get ( FoundCount ) ]