For those of your getting the ceiling mount that is listed here, you will need to buy additional screws (10mm M3*3 screws for my HD23) as the ones supplied do not fit. Also, you will need to remove the rubber feet of the projector and screw the mount into their now empty sockets, as it is near impossible to screw the mount into the intended holes without the projected tilting in some way or another. ALSO, when screwing the mount into the ceiling, be sure you drill the holes in a way that the two screws on the side of mount (that holds the rods in place), are parallel to the screen, as they tend to be a little loose and will tilt the mount in a direction, so having it tip forward is easy to fix.
TLDR; just get the mounts made specifically for your model so you don't have to hassle with all of the above.