devnul:
I'm not a financial analyst, but it seems to me that if you google payoff, amortization, net present value, etc., you'll get some standard formulae you can use as a starting point. Then you can use the hack and slash method, by playing with proportionality constants, etc., until you can match up the numbers, or you can also google for "curve fitting" and give it a try.
That could be a lot of work, however. Have you considered writing a program to simulate the operator and making your perl script do the key-entry to get the data you need out of the existing machine?
--roboticus