Here is my semi-finished project on Flickr - http://www.flickr.com/photos/45387881@N03/
. Note that it will look much cleaner once I move the cable box and the wires you see under the screen to the back of the room (just waiting on my remote extender).
So here are my suggestions from one amateur to another...
1) Get a friend to come over and help you. Stack some storage bins on top of a table (like a dresser or something), or use a small ladder and carefully place the projector at the top of the stack (as to simulate a shelf mount or ceiling mount position). Just be careful with the projector and don't let it fall! Make adjustments or move the projector around as needed.
2) Now project onto the blank wall and have a friend and you stand or sit in the same dimensions to where the seats will be... Make sure the semi-short people 5' 5" (if you care) can see over the dimensions of the front seat.
3) My personal preference is for the screen to be about 30" off the floor or so, but a little lower would not bother me. Over 40" actually bothers me, not that it causes me any neck strain, it just doesn't seem as much of a natural center to my eye as 30" did. However, I do not have anyone one sitting behind me, so I don't have to worry about that. Since your farhter back, my numbers may be meaningless, but just use your testing scenario to make your own judgement.
4) If your screen is less than 40 pounds and you do not have studs or you just want to mount directly through the drywall, then you can go buy these metal cork screw looking drywall inserts with screws that are made for picture frames (home depot sells them I think). They can hold 50 pounds each, so use 2 of them (one on the left and one on the right of your screen, depending where your screen mounting holes are). I felt pretty confident doing it like this, although my screen only weighed about 30 pounds.
For an amateur HT designer, there is no better way than to do a real test, because even if you measure it, it only takes a mistake of about 1/16" to be off vertically or horizontally. Hence, even if you pencil the spot perfectly, just pointing the drill minisculy off will result in the hole being off enough to throw the thing out of whack a little, especially from the distance you are mounting from. So if you drill one hole just barely off, then you can't make a new hole without the two holes "coming together" and forming one giant hole that is too big and then it ruins your ability to make a new hole to fix your positioning mistake. This is why it is important to get the holes drilled in the correct place on the first attempt. Of course there are ways around it even if you do mess up, depending on what methods you are mounting with.
Everyone's preferences are different, but if you just take guess measurements, you'll never know. BTW, shelf mounting is easier than ceiling mounting IMO, unless you have to worry about kids that might touch the projector. The great thing about shelf mounting is you can make small adjustments to the projectors left and right positions without relying on the swivel bar or a celing mount. Also, on a shelf it just sits there so doing maintenance on the projector is so much easier (cleaning, changing lamps, fidding with cables) than having to always take the projector down. Another advantage of shelf mounting is it is easier to hide wires and you don't have to drill holes through the ceiling if you wish to completely hide the cables (and you'll end up using less wire mold if you are hiding cables through plastic wire mold - can buy this on amazon.com).
You do need to get the measurements pretty much exact for the screen. The way I did it was I held the screen on the wall before mounting it, and had a friend move the projector until it perfectly matched, then I took a pencil and scribbled through the mounting hole until I got a colored circle on the left and right mounting hole. I was off about 1/8" vertically, but you can't tell because the black outer border around the screen blocks any light leakage. However, I should have penciled the entire mounting hole so I would have drilled it exactly perfect (instead I just marked one small dot). What I mean is fill in the mounting hole with the pencil on the wall completely so that you know it is right when you center the drill into the penciled hole.
Various sites (like buy.com) sell these tempered glass shelves that are made for mounting components under a plasama (up to 30 pounds), and they are somewhat adjustable as well. These work perfectly for a projector as long as you get the right size (they come in different sizes). They only cost $30 to $60 depending which one you get, and they look fabulous. This is what I use and not only does it look better than a ceiling mount, but it also functions better. No need for keystone, actually I shelf mounted my projector exactly perfectly to the screen placement so it needed 0 digital adjustments. The only disadvantage is my projector is only about 5.5 feet off the ground so people moving around in the back row could block the picture when they get up. However, keep in mind that most of the time someone has to walk in front of you anyhow when they get up, so this is not really much of a disadvantage.