You don't need the projector in the center of the image. If it's flipped upside down (ceiling mount) you need the projector even with the top of your screen. Do an image search on google for projector alignment .
Beyond that you could go 15% above. The keystone could give you that 15%. I wouldn't do it, but you could.
Don't install anything before trying it out with a ladder . Specs and manuals are liars!
If your buying a new projector, buy more lumens than you will use. Running it in "Eco Mode" makes your bulb last 20-40% longer and reduces noise and heat.