A few weeks ago I had an opportunity to work on a theater designed by a well known acoustical design firm. The RT60 for this theater measured at 360 ms. It had a strong slap echo and was way too reverberant for quality theater sound.
The installation had too many diffusers compared to absorbers which was both expensive and ineffective. When you hire someone to do this you should ask for an RT60 measurement be taken after the job is complete at the prime seating position. I would accept an RT60 of between 200 ms and 250 ms extrapolated from the linear sound drop from a wide-band impulse signal like a hand clap. This can be done with a Sencore SP495 using the energy time graph.
This magnitude of error can be avoided by computer modeling the acoustical behaviour of your theater based on your room materials of construction, furnishings and geometry. One method of doing this is to use the CARA design software.