Mount the screen where you want it, first. Then find the position back from the screen that is in the
middle of the manual zoom for the PJ - so that you can twist the zoom and make it both larger and smaller than the actual screen size - the last thing you want is for it to be on the edge of either of these extremes. Now find the point that is exactly the center of the screen - that is where you want the center of the lens to be. Then work out when the mount needs to be screwed in to place the lens where you want it (on many mounts this is off center from the lens) Now, like you said, chances are there won't be a roof joist at that exact point, so what you do is find the two joists to either side of this point and screw a small bit of 2x4 onto the ceiling and then the PJ mount onto this. You can tart the 2x4 up later with some edging and even paint it if you want. Here's a photo of what I did in my old room:
As you can see, it is screwed into the joists to either side of where I actually needed the PJ to be. Hope this helps!