General

For an L/D of 30:1 (a conservative estimate? Wind effects?), use 1000 ft of height required for 5 nautical miles of distance to cover this is about 1000 ft per 10 km

Where does this come from?

30/1 means from 1000 ft you can fly 30,000 ft over the ground

1 nm = 6076 ft, so 30,000 ? 6076 = 4.93 miles = 9.1 km