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I am trying to avoid using a script because the client will be using 8.5 and 9 client software as well and 10.
Go ahead and write scripts! You shouldn't have any significant trouble with putting scripts in your database as long as you don't rely on script triggers (a version 10 only feature). The scripting differences between these three versions are very small. If you want to be especially cautious, use a copy of FMP 8.5 to either design or at least test your scripts. It they work for the 8.5 version, they should behave identically with the other, newer versions.
I can't easily answer your technical question without a bit more info on how you've set up your tables, keys and relationships but can give a general suggestion:
In filemaker you have some functions known as Aggregate functions. You can look these up under that name in the on line help to learn more about them.
Sum(relatedtable::field) for example will return the total that adds up the value in the designated field of all the records in relatedtable that are related to the current table. This is often how you can get results of the kind that I think you want.