At what height above earth s surface is the gravitational acceleration reduced from

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

The power of the magnet will be stronger