Thanks to both of you dudes
I already read what you've written on the subject: this is very precious but not the "scientific" elecbyte's method
Honnestly, the question is answered not only for me but also for the development of a bg tool: the precision is necessary
I'm so sick about the disappear of this formula: I used it a lot of times but never thought to copy it somewhere
Shame on me...
Spoiler: the closest explanation that I've found untill now is here (click to see content)