Hey Rick, yeah it's been awhile - the phrase "nervous as a long-tailed cat in a room full of rocking chairs" comes to mind :=)
If you still have your copy of Everest's MHOA, it may help to re-read the section on Diaphramatic absorbers starting on page 205 (of the 4th edition) - there's a graph of 3 panel traps showing peak absorption, approximate bandwidth, and absorption coefficients (the ones with rockwool inside peak fairly narrow at around 0.7. He goes on to talk about drywall on studs. It's only a few pages.
To figure total sabins, multiply the coefficient by square footage - be aware that this is a GENERAL idea, it would need to be measured in place BEFORE and AFTER to know exact values. Eric once told me that he'd given up on a particular low frequency room problem using panel traps (musta been pretty bad) because by the time he got enough absorption the traps were "giving back" their own contribution to that resonance. He also commented that if you're looking for EXACT frequency peak you need to build them with a flexible seal around the front panel and fasten the panel with screws that can be tightened or loosened to "tune" the trap.
This type trap works by the energy conversion to heat, in two ways - first, the flexing of the front panel takes energy - this is converted from panel movement into heat, lessening the amount of acoustic energy. Then, the internal absorbent works similar to a purely absorbent trap - that is, the air being pushed and pulled thru the absorbent causes that energy to be converted to heat due to the frictional losses in getting thru the interstices of the material. For this reason, the soft fluffy stuff isn't the right choice here.
In Joe's room, I just got plain LUCKY - I won't sugarcoat it. He had a (un-quantified, as far as I know) problem at two frequencies, and a desire to keep the decor from looking crappy - I offered a solution including calculations for frequency (only), he built 'em, and only AFTER the installation did his room get "dolby certified", whatever that means - I don't know the particulars of that, only that he's a happy camper.
It seems everyone's looking for short, concrete answers to these questions - I'm sorry, but there aren't any. You need to work through each instance separately, and if you're building from scratch or retro-fitting, it helps to get a "ballpark" idea of what to expect by factoring in the LF panel trap effect of your drywall when doing predicted reverb times - the reverb calculator at John's site takes this into account when you tell it what your walls are.
My (mostly theoretical) approach to this (which seems to work fairly well, considering the number of happy "customers" at John's site) is to, if possible, keep modal problems to a minimum (mostly scratch building of course) by shying away from "bad" room ratios in the first place - next, measure the room once it's "soundproofed" - this gives a baseline so you know what to attack next. if there aren't any HUGE peaks or dips, the standard corner absorbers/cloud/first reflection points/super-fat rear trap approach has worked well for a LOT of people. This is of course for a CR - live rooms have different requirements. Longer 'verb times, NON-symmetry(for variations dependent on mic/instrument placement) and flutter control would be 3 main things in a live room.
If there are STILL unacceptable peaks or dips in a CR after the basic treatment, I start looking at boundary effects (either adding more wall absorbent at the rear of speakers, or move 'em a bit) and if this does little or nothing, if the room's a rectangle I check which modes would likely be causing the problem - starting with axials since they're stronger than the other two. A length mode needs to be treated (for panel traps anyway) by placing the panels on the SIDE walls (or ceiling) at points of highest PRESSURE, not velocity. I use Harmon's simple but graphic mode calculator for this usually.
Sorry to cut this short, but those "rocking chairs" just don't seem to wanna quit - I hope this helps some. Best I can offer for now is that this appears to be an exact science ONLY for people with bigger heads than mine - for the rest of us, it's "cut and try" more or less. Maybe my post in John's acoustics forum titled "are my speakers in a null" will give a bit more, been a while since I reviewed what I said there meself... Steve
I tried to find answers to these questions a long time ago, and Ethan, Eric you tried to explain but NEVER did answer them to the point where I understood. Heres a few more .
And they will be appreciated. 
I learned one thing. Its another country down there. NO one spoke fucking english. Left Sac the next day after 3 hrs sleep. It was a clear day, but when I hit Mt. Shasta, the wierdest snow storm clouds I ever saw engulfed me. Drove for 15 miles with my head out the window to see where I was going. Thank god I made it to Yreka and got a room. Damn, never again. Too old for that stuff anymore 
