What is the better area to post in DIY corner or Pros and Joes?
I don't know. I typically stick to the 'under $3,000 projectors' section of the forums. When I am ready to do my build, I will likely be looking for some answers.
For a baseline of building, I think the first goal is finding the room that will be used. Then I would look at the layout of the room and determine my viewing wall and start working out equipment locations. Where does the gear go, where will the projector go, where will speakers go, what works best in a general sense. Start talking about specific ideas and concepts, maybe start sketching them up. Hand drawings or photos or some online drawing tools are just fine. Maybe even ask about what online drawing tools are out there that do a decent job. I don't think much about 3D vs. 2D drawing. I just would like something I can put some furniture into and get some relatively decent floor layouts to work with.
Things can get crazy if you start looking into sound-proofing the space. Truly sealing the room from audio bleed to other spaces as well as acoustical treatments to get the best sound out of the room you are in.
The headache, at the end, is that a full and proper design may require years of experience more than just research, and that can be problematic to anyone who doesn't have that actual experience. So, you have to just roll with the concept that you will have to accept that there are things that won't work great, but you will still love what you have.
I get on the forums and I talk to projection. I don't talk to audio or acoustics as I am more practical in terms of audio and I don't really have a room that will ever be close to an audio-ideal. So, I enjoy the audio and focus (ha!) on projection. I can talk about the space and how to treat it for better projection, placing a projector, and screen size, etc. I also bring up lighting a great deal as that is one thing that almost everyone gets wrong. They under light theaters and under zone the lighting loads. As if, having 50 lights in a room is somehow different than having two lights in the room when the lights are off. They go LOWER, because, you know, it's a theater. Think about that for a minute. Then think about how dark paint on the walls and ceiling, combined with dark carpet impacts reflections and available ambient light when you want and need it. So, yeah, lighting in a theater should be doubled from a traditional room, and should be zoned heavily. At least one zone of lighting over each seating row, and another zone of lighting for 'general' room lighting. Additional zones for sconces and under riser lighting may also exist on separate zones. No lighting should be LED unless it truly is infinitely dimmable. LED lighting has very serious issues with dimming as well as incandescent lighting does.
I think when you have a room, and a general layout, then you pick a seat that is yours. That is, the seat about which the rest of the room is designed. If you want two rows of seating, maybe it is the center seat in the second row. Maybe it is the center seat in the first row. Then you design around that for audio and video. Base your screen size on that. Base your audio positioning around that. Then make what allowances and compromises you would like for the other seats in the room and the second row.
Movie theaters have to do this. You simply can't make an ideal situation for every seat and every location and every circumstance. I think that's the thing professionals get and amateurs really struggle with. They think that pros have figured out a secret that they are missing. When in fact, pros have let go of it completely and just design around a primary location and don't worry that much about the rest.
So, practically speaking, pick a seat, design around it, do a good job with your room colors, do your lighting properly, and run conduit to the projector location from your equipment location which is NEVER near the front of the room where blinky lights are visible to distract.