I don’t know anything about 50ft, but for perf A jets the normal screen height is 35ft which is reduced to 15ft if wet or contaminated as a concession by the authorities.
The way they tie in with V2 is that the speed must be at least V2 at the 35 ft screen height, but for a 15 ft screen height, speed must consistent with achieving at least V2 at 35 ft. ie V2 does not have to be achieved at the 15 ft screen height.
S & L