At sea level, you're 6400 km away from the earth's center. Where gravity is 0.4 fewer,
in other words 0.6 its sea level value, you're x km from the center, where:
(6400 / x) ^ 2 = 0.6)
Take note that gravity is an opposite square force.
So 6400 / x = 0.7746
so x = 8262 km from the Earth's center
so you're 8262 – 6400 = 1862 km above the Earth's surface
