Depends on breeds (some are better fliers than others), but generally they need a way to get down, and flight path is generally looked at like a 45 degree angle.How high from the floor can roosts go? And We’ve decided that we’re probably going to put in a poop shelf.
However.... good fliers don't usually need that much flying space.
Chickens can hop pretty well about 1.5 feet straight up, so you can also just make a "step" for them.
I have perches at all kinds of heights. My tallest are at almost 6 feet (because my bantams REFUSE to perch lower, and I have failed at walling off where they like to perch). But I prefer 3 feet to 4 feet tall perches