There are a couple of approaches you could take. If you don't have a lot options and you don't see the values or formula changing often, you can simply use rules based on the selected answers to assign values and generate an overall score. This is the simplest solution, but doesn't scale much since it involves a little hard coding in the form.
A better solution would be to assign values to specific entries that are location in a secondary source such as SP list, XML file, or SQL. You can have a secondary data connection to the external source and the assigned value can be used to create your logic to determine the overall score. The answers can also pulled from this secondary source, so scability is much better with this approach.