Stan Brown formulated on Sunday :

And there is also mine at
http://oakroadsystems.com/math/loan.htm#Excel

Just a first look at your great work...
I'm wondering why you don't use a more precise way in calculating
effective interest rate.
If a bank offer me a loan at 6%/year and then we agree on monthly
payments, I would pay 0.486755056534305%/month, or something like that.

You have followed Excel philosophy:
interest rate/month = interest rate/year diveded by 12.
Which is mathematically and financially wrong.

Bruno