The tform files are only for the edit forms and not the lists. The lists are defined in the .list.php files, but this file contains only the definition and not the the actual list generation.
If you want to create custom list colums, you have to extend a function in the list library. For an example see e.g. the mail_user_list.php file in the mail module. As you want to calculate a specific value in every row, you will have to extend the prepareDataRow($rec) function of the listform_actions.inc.php class.