calculating percentages based on checkboxes

The <> operation should work just fine with a Numeric type field I have a number of fields that use it in calculations in my live apps. If you are having an issue with that operator I would confirm the field types of the fields in use and make sure they are all Numeric or formula Numeric fields. If you are still having issues then I would suggest creating a case with the Care team so they can take a look at the formula inside of your application and see what might be causing you to get those error messages as it could be a type mismatch that is sometimes easier to identify from inside the application.