My shower stall has a 2" pipe for the drain, running into a 2" trap, etc.
It's 2" all the way. Anyway, I need to shorten the straight piece running
into the trap. The trap was set in a little low to the ceiling below. I want
to be able to give it a little more room. What should be the minimum
distance from the shower drain to the trap below? I know gravity does the
work, but I want to make sure the drop is sufficient for the water to run.

Thanks,

John