Stan Brown used his keyboard to write :

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.

It's not "Excel philosophy", but the standard way banks in the US do
it.

Ok Stan, may I correct my previous statement:
Which is mathematically wrong and financially correct in US,
mathematically wrong and financially wrong in Europe.

BTW Stan, I'm going to study your work with the related literature from
your site.
Then, if you wish, I can send you my better-conceived remarks.

Bruno