I remember in my UAS day min turn back for the bulldog was 500ft and only if there were no other options, cant remember if the aob required was 45 or 60 deg. i was definitely shown several at various heights by the cfi not sure if down to 500 ft. we certainly practiced at altitude down the center line. again hazy memory but think think with practice 600ft was doable with a safeish margin. 500 would have been wings level at 50ft if you executed perfectly. most if the time i'd have turfed in.
but there have been places, such raf woodvale, were a efato at 300-400 is going to be painful! 21 at daymns hall not nice either!