What alternatives do I have to use the Computed Field module? I am currently using the Computed Field module to calculate field in Field Collection and use PHP code to generate the formula. The problem is that the user does not enough PHP skill to write that PHP code. So are there any alternatives that allow users to choose which fields and calculations instead of using PHP code?
Using rules you can do basic calculations with field values and write the result into an other field. If you do the calculation after an instance of your content type has been saved you make sure the value is always up to date.

Of course you can implement your own action as well and hook it into rules. See hook_rules_action_info.

Whether your users find rules easy to manage and you can trust them to give them access to the rules system is an other story. Depending on what you want to evaluate you might just add some fields to get all the information you need and add a custom action to do the evaluation.