You do not count vertical distance while jumping down, though hypothetically you need to displace at least 1” for a 1” base to move off of a ledge. Ever so slightly more for the larger
SM bases.
And you can jump down as far as you want, but once you reach 6” drop you star taking mortal wound rolls. You could drop 18”, but you’d take 5 rolls. No mater how far down, it doesn’t count for distance. Makes assault troops have a reason to climb buildings to avoid counter-assault.