I'm curious as to the notion of "close enough". i.e. how much tolerance is there in order for those ratios to still be useful?

To me, "close enough" is just taking:
8' x 12' 9.6" x 18' 7.68"
and bumping it to
8' x 12'10" (or maybe 13') x 18'8" (or maybe 19')

Again, not sure exactly what that will do or how much impact that will have when deviating from the ideal rations, but I think that if they are that close, the stuff IN the room will have a bigger impact than round up a couple of inches.

that's what i think too. a couple of inches won't matter really, except if you got "golden ears". ;-)

My old avatar....

Looks like someone needs some advice. Fast.

Guys, quick! Help him before he hurts someone, or himself.

??????
Advice?????

Nick - you might want to look at seating first before committing to a room width. At 12 - 13' wide, you will be pretty much limited to three chairs across the room and still have enough room to walk around them. If I ever build another HT, I plan to build it 16' wide so that I can have four seats. The length will be around 24' so that I can have a second row of seats and enough distance from the seating to the SB speakers for optimum sound for those channels. 24' depth will also provide enough distance for wide channels or a second set of surround speakers, if I decide to go that route.

We have a space that is 17.6' wide between the stairwell and outside wall (already studded). I need to have a hallway in this space as well, and I don't want the hallway to be less than 4 feet wide (so it doesn't feel cramped) and I need 6 inches for the staggered stud wall. That leaves me with 13 feet max for width. Nothing else I can do about that.

Why do you need a hallway? Priorities, man!

Yea, I know. But the alternatives are:
A) have everyone walk THROUGH the home theater to get to the rest of the basement

Or

B) have everyone walk AROUND the house and come in the back door to get in the rest of the basement....
Hmmm......