How do I find the right polynomial to be included in the xml file to bind the EPR and the n1?

This may help - http://www.fsdeveloper.com/forum/threads/a-gauge-to-fake-epr-and-egt.428293/

I mean the formula to correct the value of the EPR as a function of n1, in the xml file of the gauge.

No one wants to write your code for you. It takes a lot of work to figure out the formula for each engine, no one is going to do this for you.

 

If you want a class in math, that's probably not going to happen either.

 

In short... you really are going to have to figure your own way out on this one.