Are there any in-depth docs for the calculation field?

I am trying to build a form that calculates the amount of two fields and then adds on a fixed amount and then passes this info to Paypal.

Here are the details.

We have lodges that pay dues of $6.00 a year per member.

I need to understand how to use the calculation field to add one field that varies - the number of members

then multiply that amount by $6.00, which is a fixed amount not editable on the user end of the form.

This amount then adds another fixed amount say $2.00 for a processing fee and then this total gets handed over to Paypal.

I have been looking for docs to understand how to do this but I've been unable to locate any.

Help in doing this would be greatly appreciated.