Since I haven't received any responses I will assume that I am on my own. I am certain that someone has calculated or measured the reflection characteristics of a wall/plane and has determined a ratio of treated to untreated area in order to achieve the desired reflective reverb energy. I guess I will just take a guess and go from there. I was hoping to prevent multiple iterations of the placement of treatment on my ceiling. I don't want it to look like swiss cheese and cost more than necessary.
The answer is you have given us minimal data to help you.
I will say that most of this talk about treating rooms is "BOGUS"!
In general it is much more a speaker problem than a room problem. Now some rooms do need treatment, but the solution are empric and only trial an error will help you.
in my experience ambient rooms can still sound very good.
The speaker issue comes into to play when the off axis response does not closely mirror the on axis response except for a gentle smooth roll off at the top end.
Most of these problems arise because of drivers being driven into their break up range where they really start to beam. Tweeters can also be problematic. So if you have an ambient space then you need to select your speakers with extreme care.
In our former home these speakers were in a very ambient space, but sounded excellent.
In our new home they are not is a space with a discernible echo. But I am honestly uncertain as to where they sounded best.
I personally have never placed any so called "sound treatments" as such in any of my spaces. For one thing they often look awful and I don't think my wife would tolerate them for an instant.
However, I do use furnishings and and decor to my advantage. But none of my rooms have looked obviously "acoustically treated". I also use room dimensions when I had the luxury of a new build in our current home. So the main AV has optimal dimension ratios and a rear that is relatively dead by careful design, but there are no treatments as usually understood.
