In a database environment we can create a query that asks questions of data stored in tables or other queries. In this Revit is a little different. We can ask questions (queries) of tables (element categories) all day long but we can't ask questions of the answers to previous questions...or in database terms a query of a query.
There is one example in Revit that is already wired to permit such a result, a percentage of another value, such as Room Area. In this example I've made a little tiny project, created a schedule and added a calculated value to the schedule. This is the final result of the schedule.
This is the properties of the calculated value.
Any parameter's data type, based on what you include in the schedule, and tell Revit to Calculate Totals will be an option for consideration with "Percentage" selected. For example these are my choices if I also include Perimeter and Volume in the schedule.