vertical sleep space - minimum?

Did a full-sized version of my profile on rosin paper and realized the upper bunk is REALLY close to the ceiling. I need to bring that down, which means lowering the windows and probably the bunkette (benches). Question is - how much?
Looked at plans in the design library that include bunks - saw the 2+2 is something just under 24" each bunk, but recommends going to 5' for something just under 30" each. Campster doesn't have specifics, but doesn't look realistic for anything but a really small child, anyway. I'm hoping this camper will last into the teens.
Upper bunk is 75x56 to accommodate two children, average height (it sits under a curve) from bunk surface to ceiling is 20 1/2" -- I think not enough. But how much is enough?
Looked at plans in the design library that include bunks - saw the 2+2 is something just under 24" each bunk, but recommends going to 5' for something just under 30" each. Campster doesn't have specifics, but doesn't look realistic for anything but a really small child, anyway. I'm hoping this camper will last into the teens.
Upper bunk is 75x56 to accommodate two children, average height (it sits under a curve) from bunk surface to ceiling is 20 1/2" -- I think not enough. But how much is enough?
