Hello, I am new to all of this, so please be patient with me. I am in the process of building a dedicated HT. My question is, how do you determine how far back and how low/high to mount the projector. I am going with an Epson 1080p projector (6100 or hc1080). I am not sure which ceiling mount to buy.

The manual of the projector will have the specs. For throw, there will be a ratio, say 1.4-2.4x. That means you take the 16:9 screen width, and multiply that by those numbers to find the range. For instance, an 8' wide screen using the above example is 11.2'-19.2' back. That would be the range one could put a projector with that lens.

The amount of drop depends on a few factors. First the offset of the projector (some, like the Epsons, have a shift). Also you have to know how high you want to mount the screen (thus how low the top of the image will be from the ceiling). Once these 2 things are known, you can calculate the offset.