It is my understanding that the original 50 feet height was somewhat arbitrary, dating back to the early US days of aviation. It was based on the height of trees around a particular military compound which in turn was relevant to a particular military aircraft marketing demonstration (if my recollection is correct, by Curtiss).
A number of the early rules were plucked out of the air on the basis of engineering judgement to get the US civil rule making machine started.
I have never seen a certification justification for it but, if you do the sums for, say, the 707/DC8 - ie the early jets in vogue when the present ruleset took wing, with 15 degrees bank the outer pod drops down around 15 feet so I have always presumed something along those lines to have been the basis for the 50 feet turning clearance to the net flight path.
As such the original screen heights are not, in any way, sacrosanct, just historical ...
[ 30 November 2001: Message edited by: john_tullamarine ]