Mary,
I don’t know how you would read the color values ‘live’ during a shoot, other than importing a test shot and taking a frame capture and checking the rgb.
I looked at some of my chroma footage and found a range from 103,190,117 on the bright side to 71,141,83 on the dark side. This was all from one frame on one camera. I don’t recall having too much trouble keying this. I use New Blue’s chroma key instead of the one supplied with Vegas.
One thing I can say is that you want your screen lit as evenly as possible. Mine is cloth that hangs from 9 feet up, curves at the floor and extends about 12 feet from the wall. I use 3 -500 watt halogens with a softener on each. They are about 3 feet above my screen and about 2.5 feet in front of it. My talent is about 6.5 – 8 feet in front of the screen and lit from 2 points. I do not use a hair light or a back light, because of the huge amount of light already behind the talent.
You want to be careful of shadows because they can be hard to work around. I recreate all my shadows from a silhouette of the keyed footage.
I hope my 2-cents worth helps you in some way.
Jeff
http://www.narrowroadmedia.com