Originally Posted by MississippiMan
With a 12' Throw distance, you can get anywhere from a 97" to 106" diagonal image. 102" diagonal seems a good middle ground.
White walls, eh? Ugh. Your still too sparse on the room's dimensions. Things like Ceiling height, and width across the Screen wall are also important to be able to judge just how much of potential problems might be possible because of projected light reflecting off such surfaces. A PJ's ("projector) own light output can work against itself if the light bounces back onto the screen, or lights up the entire room just as if you had lights on.
So I'll speculate on a few things to keep things moving.
Say you have a 96" high screen wall. A 102" diagonal screen is 50" tall. That leaves 46" to work with between the Top & Bottom edges of the screen. Your mentioning a 12' seating distance seems to indicate one row of seating. OK....then in that case, you can place the screen as low as possible to increase the distance between the ceiling and the top of the screen. That is a good thing to do with a White Ceiling.
Let's say you place the screen 20" from the ceiling, leaving 26" to the Floor. That's pretty close to being "centered" on the wall, and 20" distance from the ceiling will help some to reduce the amount of reflection.
The side walls are nothing if not the same issue....adding to the deleterious effect the ceiling can have. If the side of the screen comes in too close to the walls, light is gonna bounce off 'em with a vengeance if a bright scene is playing. For many, (...putting any reduction of contrast aside for a moment....) if they are watching a movie in a darkened room and the scene goes from a dark to light one, having the entire room light up like someone flipped on the lights can be in the least disconcerting, and often it's quite annoying....seeing that it can whip one right out of being immersed in the image/story.
So what to do? For some the solution has been to possess a High Gain Retro-Reflective screen that has a narrow viewing cone that restricts light to an area centered on the PJ / Seating area. That's OK in a room where the screen is almost the same width as the room, and watching from a side angle ain't a'gonna happen.
For others, owning a High Contrast, dark Gray screen works by mitigating the amount of contrast-related damage to the image all the reflectivity can cause. Blacks go "Gray...Colors get muted....shadow detail
s get lost.
You have a PJ that when in "Torch-Mode" (full brightness) can light up that 102"er brighter than a Plasma/LCD TV. It's a DLP, so that's good....but a highly reflective darker shade of Gray can help fight against weakening Black levels and keep Colors looking dynamic and richly saturated.
Also, when you do view Movies / TV at night, running the PJ in "Low Lamp" mode can help contain reflected light. What does get loose and bounce around will....when it get's back to the screen, fail to affect the screen's performance nearly as much due the lessor amount of luminosity available to be spreading & bouncing around the room.
To be frank, there isn't a white screen in existence that can really fight against ambient or reflected light without being grossly high in Gain. Even then, the PJ must have adequate lumen output so it can power up a decent image on a darker Gray surface.
There is no way you can provide too much info...or too detailed an analysis of your situation. Many such as myself can take all that info a precisely tell you what you need to obtain not just "passably good" but downright stupendous results, But....it's info we gotta have lest our postings be nothing but speculation or generalizations.