Roosts in my coop are 18" ans 36" from the floor and the girls love them. They are also above the nestboxes, not vice-versa. Higher roosts ensure the chooks won't sleep (and poop) in the nestboxes. Others have done the opposite but I have read in a few sources to put the roosts higher.
It depends on your chicken's breed, too. Silkies can't fly so their roosts need to be jump level. The big heavy breeds can hurt their legs jumping down from high roosts. But my bantams are able and agile fliers, and they enjoy a higher roost. My roosts are around 4 1/2 to 5 feet high...and they'd roost higher if they could. I had to get one down off the roof of our garage yesterday!
By the way, flatter roosts are better for a chicken's feet, so don't use an actual round pole.