I also tried Job Numbers as the break field, as well as Job Code (which I previously failed to mention -- Job Code would be a code attached to each client, under which you'd find these Job Numbers. For instance, there may be a Job Code of XX with Job Numbers of XX001, XX002, etc). Neither of these seemed to work.
To use a sub-summary value in a calculation you must use the GetSummary() function, and the breakfield must be the same field as the one defined for your sub-summary part. In your case, it would be:
GetSummary ( sum.net, Job Number )
Then you can divide the result be another value, e.g.:
GetSummary ( sum.net, Job Number ) / AnotherTable::Customers
I didn't get how your tables are related, and what exactly is the Customer field - make sure that the reference does return some valid value, otherwise you'll be dividing by zero and get a question mark as the result.