Editing-personalizing process, adding arithmetical operations

Hello, first of all thanks for your amazing project.

i have an emonth set up with emonbase,  my goal would be to take data from a feed, apply some operation them  and log the result in another feed.

i have tried to modify the process_model.php in var/www/emoncms/Modules/input/ in order to have different possible arithmetical operations on feeds, but it seems to not apply the modification or affecting the operations on the http://emonpi/emoncms/nodes/view page.  

so where are the operations parameter stored in order to personalize them?

elsewhere is there any resource where i can find this answer by myself?

Thank you all in advance.