I don't have an answer for you as such, just thought I'd point you to another IRR custom function at the same site in case you missed it.
I have been looking for an XIRR function for some time, but have so far not found anything I could use. I am considering making an attempt to make such a function for Filemaker.
Hi, did anyone figure out how to implement XIRR?
I'd imagine the formula to be the cornerstone of a lot of financial packages and very useful.
Please reply if you have done this or have any guidance.