SOP Blank Invoice needs to show deposit/credit card payment and remaining balance

I cannot believe this is not available out of the box - I have a client that needs to show the total paid against an invoice and then show any amount still due.  I read somewhere to create calculated fields F/O Deposit and F/O Remaining Amounts, however, nothing displays on the F/O Deposit when I create these and the for the remaining amount the only field I can see to attach to it is sub total remaining - I need the on account amount to be equal to the remaining amount of the invoice.