Your studio is a big box. It may be an oddly-shaped big box. It may have doors or walls where you don't expect it. But it’s really a box.
This is a basic, non-axial Sabin calculator for RT60, the length of time in seconds it takes for a signal to drop 60db. For a recording studio, this should be about .20-.40s at 1000Hz, for a vocal boot it should be < .20s. A home theater or music room has slightly looser tolerances.