Maybe I'm just a softy, but I recently assisted a client in Germany (a recording/mix/post suite) who had hired a designer and was left with a horrible result. I offered an agreement, once I had a good understanding of his factors, of 50% to engage the process and 50% on success (and we defined what the exact acoustical goals were).
Guarantees are tough to offer, but real expectations aren't. I always want to feel that the "trust exchange" is an equal relationship between myself and my clients.
As for markers, some of this is relative to the background noise level. If the room is has low noise floor, NC25 or lower, I feel that a lower RT60 like the .2s mentioned can feel very comfortable providing you have "preserved" upper mid/hi energy (through strategic use of reflections and diffusion). If the room has a more typical background noise level, I agree with Nyal that .4s is right on target.
- LF resonances above 30Hz should not exceed 400ms (300ms preferred)
- Early reflections should be down 15dB from direct arrival for as long as possible (10ms absolutely minimum) to allow the source content to reach the listener without coloration.
Room shape/dimensions, construction systems, HVAC, speaker type/location, listener location, room finishes (which include absorption, diffusion and reflection) are ALL part of the acoustical factors that should evaluated to reach a desired response.