[marciero]

Defining "space" in terms of gravity is problematic because the force decreases continuously- no abrupt cutoff. Hence arbitrary definitions.
As stated in the article, no significant difference in gravity at 50 miles.
Again using 4000 miles as earth radius

50% gravity at about 1600 miles from earth surface.
10% at about 8600 miles

One is never outside the influence of gravity due to any heavenly body, so "zero gravity" does not exist. But you could think of zero gravity as where the gravitational effects of various heavenly bodies cancel each other out. For example, earth and moon would pull equally if you were about 215 thousand miles between earth and moon, assuming moon is about 240,000 miles and radius about 1000 miles.

So even at the space station (altitude of 254 miles) the gravity is still about 88% of earth. It just feels like zero gravity because both the space station and occupants are orbiting