As far as i know, thereīs no such thing as the perfect rgb-value for greenscreen.
The most important facts are that the screen is evenly lit and that thereīs a clear difference in color to the forground object.
In fact the screen could be any color that differs from the foreground. it used to be blue for Film, because of the way it was processed and because itīs the color that is opposed to the reds of skintone...
Nowadays when shooting digital itīs mostly green, because video is most sensitive in the green channel.
So from my point of view, you donīt have to aim at certain rgb-values in postproduction, all that matters is a clear difference to the foreground subject... :)
Of course it should not be too dark, because youīll need as much information in the green as possible to seperate the foreground...
When your keying greenscreen it is best to have your screen as even as possible so that you dont have to break up your characters into a bunch of little pieces. If you working with HD which has 4:4:4 colour space it is much easier to pull a key because there is no compression which is compared to just normal DV which has 4:4:1 colour space. RGB values depend on a lot of things...the people the objects and of corse the cleaness of the screen itself.
Why wouldn't they just ask for a true alpha channel?
Color keying computer generated graphics is no where near as accurate as simply using an alpha channel. Especially for the all-important edgepixels which should have degrees of partial transparency.
Of course, "the customer is always right" and you have to deliver what they ask, but I'm just curious what the circumstance is that wouldn't simply use alpha channels in the graphics?