Nikon D300 Complete Manual page 543

Recommendations:
1. Personally, I'd stay away from altering the matrix meter
system. Because the matrix metering system already dials
in secret compensations based upon the pattern of lighting
and the colors of the subjects it sees and the scene it
thinks it recognizes, you really would be adding meter
compensation on top of an unknown. I've seen situations
where the matrix meter gets the exposure dead on and
others where it misses by a half stop or so. So if you dial
in a half stop compensation, case one is now off by a half
stop and case two is "fixed." Did you gain anything? I
think not. If the matrix metering system is inaccurate for
you in some situations, use one of the other metering
systems instead.
2. If you always spot meter off of a gray card, consider
dialing in the appropriate compensation for the spot
metering system to compensate for the slight difference in
the way camera meters are calibrated. Note I said
"always." This Custom Setting would always override the
metering null value, so you have to meter consistently for
this to be of exceptional use. Gray cards can vary as much
as a half stop from what would be middle gray
(128,128,128), and if you spot meter with multiple
cameras, they can easily be different by as much as a half
stop due to manufacturing tolerances. This is one way to
compensate for such differences.
3. If you don't use a gray card to meter but find that you're
consistently dialing in a specific exposure compensation
in all situations, consider using this custom setting to
adjust the camera. The ISO standard for meter calibration
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 543