How I can use calculations with Form Processor Pro ?

You can use calculations on any pages started from the next from where needed data was entered. Just make sure you have separated all variables and signs by a white space. That's the only one rule.
 Example:

You have three text input fields:

You can use following expression on the next form page, in the database template, log file template or email template.

If field_a=10, field_b=5, field_c=3, then this expression will return:
45