I'm assuming you are using a digital PJ since you are employing a grey screen. After that I think one needs to consider the front row seating position as well as the room look. There are those who would argue with this but I don't like the look of a room that is all screen. That said, then I think you need to consider the front channel speakers. If you intend to perforate then this isn't an issue however since you can place them behind the screen. I personally like the look of rooms with screens that take up somewhere between 2/3 to 3/4 of the front of the room. Smaller tends to look a little lost to me and larger gets into taking up too much room for esthetic reasons.
Another consideration is pixel structure and minimum seating distance . I'm using CRT based projection so I think I'll have to defer to someone with experience to make that judgement.
I plan on using a DLP projector (Sharp 9000)
I will use a progressive DVD (TBD)
I plan on having 2 rows. One will be 12' back from the screen and one will be 20' back. If I use your calcs I should use a screen to accomodate the front seating position between 96" and 72". If this is correct I will probably go for a 87" Stewart Luxus Deluxe ScreenWall using GrayHawk or FireHawk screen material. What do you think?
Next question, how far back should the projector be from the screen assuming an 87" screen? I need to know this because I have to have my electrician put in an outlet in the ceiling. Should the outlet go behind the projector? I am putting in a 2" chase so I can run any wires from the projector to the equipment closet. Anything else I need to consider?
From what I've read in the HT Constructiopn posts, the acoustics for your second row of seating will be better if it's out from the back wall a couple of feet. Don't know if that's compatible with your plans, but five or six feet between rows instead of 8 should still give you enough leg room.
I agree with the distance between rows being a little on the high side but the type of seating can effect the need for wider or more narrow rows. My platforms are 70" and they really just make it when the seating is reclined. If you are using the comfortable couches and foot stools of some type you may need that much. On the other hand if you are using standard theater seats of some type you would need even less than I use. I got out my tape and found my screen to be 9'8" from ones head. I'm using a 16x9 107" wide screen. This quoted distance is my front row with the other two each 70" further back after that. Many say this is close but with HD or even reasonably good DVD transfers I sit in the front row and I find the quality to be fine and certainly more engaging.