FL 650 (and geometric hight ++) pan as a flatcake.
Perhaps it's a perception thing? Some people are saying they can see it at sea level others are in the flat brigade all the way up?
I have some pictures that 'show' the curvature of the earth (the SOP bolt the camera to the glareshield 'over the shoulder' jobs), one was taken at 370 - It certainly didn't look curved at the time though.
"Fascanating.........."
MT