I'm trying to create an invoice database where I have one project with multiple invoices. I need to start with a contract amount and each invoice bills down from that amount until zero. Seems like I need one table for Project Information including the job name and contract amount, then a related table for Invoices. I need previous balance, previously billed, new balance to auto-calculate. I don't need multiple jobs in one document. Can anyone give suggestions for the structure of this, including the sort of field calculations that would be best?