During training the recommended altitude using the PX4 was 800 feet...however the relatively newly introduced ULLP allows a height of 400 feet or so.
During the war the operational jumping height was around 400 feet but I know of members of the 4th Parachute Brigade who jumped onto DZ Y Ginkel Heath on 18th September 1944 which was a hot DZ from around 250 feet. Around this height there is just enough time for the chute to deploy correctly. Remember that in those days there was no reserve. However to be strictly correct this guy did say that the DC3 started at about 400 feet but was in a dive and he was tail end charlie....