I know I was very surprised the first time I sat my old Lance in my friend's '15 F250 4 door, how much cab roof to camper clearance there was on just a single sheet of 3/4" plywood, so, yes, I also agree that every case will be different and your comfort zone will most likely be different than say mine.
One thing to keep on mind about having the cabin over roof close to the truck cab is when setting the camper in the bed. I might do it differently than other folks do but, I get my camper lined up and backed under and just touch the front bumpers against the front if the box and pull away just a tiny bit and then lower the camper into the bed with the nose LOWER than the backend if the camper until it's almost touching the bed then back the truck against the bumpers and lower the back and front all the way....
Key words for me is 'front is lower than the back when loading'. With limited cab to camper clearance that could become an issue. Especially an extended cab over and a 4 door (long cab) truck.
Food for thought...
