I agree with Marcy and Koustubh. There are so many variables, all of which are driven by a specific customer's implementation and usage, that go into to figuring out or projecting growth ratios. Additionally, there is no built-in calculator within DevTest that allows a customer to input the hundreds of variables required to produce an estimated monthly/annual database schema growth rate.
The Oracle schema that the Registry connects to is the schema that will experience the greatest amount most growth. The DevTest database connection properties will identify the SID and so on if you are seeking to find where the Registry database schema is located on your Oracle instance(s). You can use a third party product such as DBVisualizer or Toad to view the DevTest schema, review the tables, and examine the size of each individual table.
Is it possible for you to ask one of your DB Admins to create some type of monthly schema usage report for you? You could use this type of reporting to create a rolling month-by-month analysis of consumption and project future growth based on your actual usage and experience. This is likely to be more accurate because it is an experience-based calculation rather than a generic calculation.