Don't worry about having the green background fill the frame.There is NO relationship between how the background is lit and how your actors are lit.I try to have the green background display between 40-50% level on the waveform monitor. The green background should be lit smoothly, both from side to side and top to bottom.Paint is always better than fabric avoid wrinkles and folds. The green background should be as smooth as possible.In general, don't cast shadows on the green screen.This avoids light from the background 'spilling' around their body or shoulders. Actors should be at least 10 feet in front of the green screen.

First, the best thing you can do to improve the quality of your keys is to improve how you shoot them.