That depends on the throw distance of the projector you are getting. Have you chosen the make and model? If so, projector central list the specs of most projectors and is a great resource for this information (throw distance etc).
At the time of the construction of my home two years ago, I did not know which projector I would be using. With nothing to go on, I chose to have the location at 15 feet back from the front wall. I had looked at some specs from various projectors that I was considering and the two top models would work from this distance. You need to figure out what size screen you are going to be using as well as this will affect the location of the projector. My room is 25 by17. I'm using a 106 inch diagonal 16:9 screen which isn't a bad size for your room either with an Epson LCD projector. You did not mention this but I would also recommend running a pipe,pvc, from where you plan to mount the projector to where your equipment rack will be. This will allow for cable changes if the standard for video connections changes. In the last couple of years it's went from component video to DVI to HDMI.
All good points bulldogger. In the case where someone maybe new to the hobby, I think it serves them well in the long run to hire a local HT installer the first time. If the right person is hired they will make sure that the layout is done for future considerations, along with being right the first time. But that doesn't mean you close the door and say go to work. I would want the installer to explain what they are doing and why, so that I am learning along the way. I used a local person the first time and volunteered to be his second hand...saved money that way, since it was a two person job. I learned every step of the process, so when it was time to redo the work for another upgrade....I did the work myself.
If you can find a long throw projector that meets your screen size needs, I always think putting the projector against the back wall is the way to go. No wires and no hanging projector in the way. You can even hide it if you wish.
Keep in mind however that many projectors are not that long of a throw, so you have to decide which is most important to YOU.
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