Cardas/Fibonacci ratio is fine, but recognize that the perfect room doesn't exist. The RT60 of all that makes up a "dimensionally perfect room" could make a perfect room terrible. RT60 measures reflection and absorbtion of different materials. It is the absorbtion and reflection that makes your room yours. Like dressing yourself in the morning is your taste, so too is the constructing of the room to give it its own personality.
Imagine two identical dimensional rooms:
1 has no furniture except a chair for listening $987,654 of the finest audio gear delivered to hard flooring and nothing to absorb sound other than the ceramic tile covered walls and ceiling...likely this room is live and harsh with to much reverberation
Now imagine the same dimensioned room with a comfortable sofa, plush carpet, beautiful drapes, paintings on the walls and wall treatments to absorb unwanted first reflections and diffusers behind your listening postion to help develop sound staging and imaging...likely this room will be tolerable with $1,000 worth of gear.
Bottom line dimensions can help make a room good but until it is dressed to suit your sound preferences the bazilllion dollars in gear will never stand up to expectation.
