How do you calculate the earth's gravitational pull on astronauts?
F = GmM/r^2
just like anything else
Now, normally we just absorb most of that into weight = g*m where g = MG/r^2 and r is the radius of the earth.
If your astronaut is at R = 2r, then his weight is 1/4 normal. At 3r, it's 1/9, etc.