Complex sentence: After making an allowance for the differing altitudes of the aircraft, the difference in sunset times between the aircraft and Norfolk Island allowed the crew to determine that the Cessna was south west of the DC-10 by about 400 nautical miles.
Split into simpler sentences: After making an allowance for the differing altitudes of the aircraft, the difference in sunset times between the aircraft and Norfolk Island was also noted. This data allowed the crew to calculate that the Cessna must be south west of the DC-10 by about 400 nautical miles.