This sounds like a middle school or high school geometry problem.
A=15ft. The distance from the screen to the beam
B=10". The distance the end of the beam to the ceiling.
X=4A/3. The ratio of the distance the projector is from the screen compared to the beam.
10" * (4/3) = 13.3".
So make sure your lens is 13.3" below the ceiling and you will have line of sight. If however, your screen is not at the meeting point of the wall and ceiling, but is instead a couple inches down, you won't have to have the lens as far down.