You can't really offset the projector too much off center. There is a dial that allows the image to be tilted up and down, and left to right, but if it all possible, try not to do that since it does affect the image quality. The closer the better, you want to maximize the light output. The quality of the image will depend more on the room controls, such as wall color, reflections off of surfaces, and obviously ambient light. The more control you have, the better the projector is going to look. What you will notice is the more lights you have on or can't control, will adversely affect the overall contrast. I have 10 foot ceilings as well, and in my case, it works perfect in that I had about a foot drop from the ceiling to the projector mount. I didn't have to use either of the position dials. The Epson is flexible compared to others I've owned. As far as how far back you can sit from the size you want, only YOU can decide that. I wouldn't buy any screen until you get the projector. If possible, just project it on a wall or sheet to see what is comfortable for you and the family. People can throw any size they want, they just have to like it.......give yourself the option to choose. I thought I wanted a huge screen too initially, in the end, 87x49 was more than big enough at my viewing distance. As far as mount, I bought one from mono price, then modded it with hardware from home depot. Mount was $10.00, and more than does the job.........just depends on how handy you are. There are videos on youtube that easily show you how to build your own with about $5.00 in hardware from Home Depot or whatever is near by. Good Luck!!