ProjectorCentral also shows zoom range as 10'4" to 16'6", so I don't see the disagreement.
Placing a projector at its closest position will allow the most light out of a projector. Placing it at its furthest will allow less light out. But if you will only be watching it at night, then brightness won't be an issue anywhere from mid-position to closest. I've never noticed a sharpness or focus issue with my JVC projectors at either end of their zoom ranges but I can't speak to BenQ.
Your W1500 does not have a motorized zoom, so zooming in and out to make a 'scope film fit your screen height is probably not a goal, right ?
Mid-position will be right over head, and further back will cost you some brightness. I would plan on 10'6", but I always want the brightest possible image.
I suggest you don't mount it yet. Wait until the screen is up, then try several nights viewing with it on a table at various distances. Keep in mind that bulbs lose brightness over time, so if the furthest distance 16' seems barely satisfactory in brightness now, you will be less satisfied later.
Of course, you could always get sneaky and install a plank on the ceiling, the projector mount to another board and the board to the plank via a drawer glide. That way you could slide the projector over the whole 6' range of motion.