I have an 11 foot ceiling and things are mounted just fine. Chief brand projector mounts accommodate a standard size pipe (think it's 3 inches diameter). It is just a matter of going to Home Depot and having them cut and thread a length of pipe. I made a pretty strange PVC contraption that allowed me to make an adjustable length tube, got the length measurement and then measured what was the final setting. Sane people probably just use some kind of normal measuring technique.
The AVS folks have good pricing on the Chief mounts, they're who I got mine from.