You need one or more additional tables and you'd store the contracted amount data there.
The simplest setup (which probably won't work in the long run, but helps me explain what you need and why), is to use this:
Contractors::ContractorID = Payments::ContractorID
You'd store the Overall amount contracted in the contractor's table instead of payments and then you can access different figures for each contractor. A calculation defined in payments can use GetSummary to access the payment subtotal so that you can then compute either a percentage or a total figure for that contractor.
Where this may fail is that the same contractor may contract for more than one project. If so, you'll need a table for your contractors and a table for the contractor's projects or contracts. In the payments table, you'd then need to specify not just the contractor but the correct contract or project record...