Projected Effort Variance % is already defined as you have indicated.
Schedule Variance % could be configered, all you have to do is complete the Display Mappings.
From the Object Status Report, you can the create two New Virtural attributes using the two required attributes on the Project.
The Overall Status (which is a calculated number attribute) can then use these two new Virtual Attributes as part of its forumula.
But saying all this, good luck with getting the required results you are after as you will be trying to determine the Overall Score based on two percentages and a 'rating' which can be a value 10,20,30. In addition, even if you were to correctly use these values, the Overall Status would change as the project progresses as the '%' fields will automatically change over time. You may want to consider a process that will be pick up the attributes when the user creates the Status Report, and to store them as values of 10, 20, 30 (which is how the Scope attibute is stored), and then use these now 'fixed' attributes to calculate Overall Status.