If the software is written and being tested, why do you need an estimate? Just use something like David Wheelr's slocount to count the actual code.
Have you googled for "function points" or looked at wikipedia?
http://en.wikipedia.org/wiki/Function_points
Basically, function points are about external things that provide function to the user. It is implementation independant.
Data Functions → Internal Logical Files
Data Functions → External Interface Files
Transaction Functions → External Inputs
Transaction Functions → External Outputs
Transaction Functions → External Inquiries
But, since you posted this to Meaningless Drivel, you may already know that. A lot of people think that all of Function Point Analysis is meaningless.
See “An Empirical Validation of Software Cost Estimation Models”, Communications of the ACM, v. 30 n. 5, pp. 416-429, May 1987.