Calculating gross monltly payment

Hi all
I'm looking for a permanent position in Switzerland.
I got some good offers.
What I can't understand is how to calculate the gross monthly payment starting for the yearly gross payment.
Do have I to divide by 12? or by 13?

And also I'm trying to understand the approximately net payment thanks to http://www.lohncomputer.ch/
Do you think that the estimated net salary from lohncomputer is correct?

Thanks in advice