Hi Rob,
I did something similar to what you're describing in the context of financial data, thought my notes might help. I had a custom form that had fields corresponding to the various budget items we needed to report to a client on - # of jobs executed, PM cost, extra fees etc. That form attached to each production job, and was completed normally by the team. Another master project existed that had the same form attached, but filled out with the total budget - how many jobs were included in the overall SOW etc. The trick in that form is that all the values were negative, so a contract with 30 deployments had a budget of "-30".
From there, I could build a report that pulled those columns for every project, sorting my master budget project at the top. When I told the reporting tool to sum the total of the columns, what I got was the total budget offset by however many projects were executed. So a budget of -30 that had three projects executed showed a remaining budget of -27.
It wasn't perfect, but it met the need in the moment since I couldn't find a way to tell the reporting tool to subtract one group of items from another in quite the way I wanted. I'm betting there are more elegant solutions with textmode, but it was not in my bandwidth to figure out at that point.
Hoping this points you in a possible direction?
Katherine