Apple Final Cut Pro 7 - Manual (Page 1238)

Note: You should remove the majority of the green screen using the top handles, but
stop at the point where there’s still some fringing around the areas of fine detail in the
foreground subject. This is the point where you should start using the bottom handles
to expand the keyed area more softly. It is not important, at this point, to completely
eliminate this fringing, only to make sure that it’s soft and translucent, rather than pixelated
and solid.
1238 Chapter 72 Keying, Mattes, and Masks