What I find with a lot of people starting out in green screen keying is, they throw the shots into the keyer without first grading them, and that’s a recipe for trouble. Your first step in editing the key is to color balance the footage against an actual scope (turn on scopes in FCPX).
When I shoot greenscreen I bring a MacBeth color chip chart along to shoot thru the lens; built-in camera bars don’t relate to the actual scene and lighting. Bring a color bar chart to the next shoot and you’ll see a huge difference when you adjust to that reference in post.
Get the whites at 100 and the blacks at zero or seven point five IRE units. Adjust the mid-tones until they look good and still retain detail. Look at the saturation and skin tone ranges. I like my greenscreens to peak at around 80-90 IRE and be well-saturated. Saturation is actually more important than brightness but evenness is most important over all.
Only now are you ready to try the keyer.
FCPX does have additional greenscreen tools in it. You’ll want to play with light wrap settings and the greenscreen compensation which applies an artificial magenta backlight – that works, sometimes.
I’m unaware of what you’re shooting on, but in the FCPX inspector, make sure you have “spacial conform” set to “none”. Now you’re working with the full native pixels you shot.