My 300s appears to be overexposing - especially in the image playback - is there a way to calibrate or check the exposure measurement? Have been compensating exposure with some success.
Automatic Exposure is never perfect, even if such thing exists.
What cameras do it attempt to keep most tonalities within the dynamic range of the camera. It does so by measuring the scene in different spots, between 1 and 91,000 depending on the camera and metering mode and then computing exposure from there.
If you see over-exposure, meaning the image is clipped at the highlights which shows as the histogram right against the right edge while there is room on the other side of the histogram, then that would considered actual over-exposure. If the histogram is also right against the left side, then the scene is beyond the dynamic-range of your camera and no perfect exposure is possible.
If you consistently get over-exposed results, check if you did not leave positive EC on. Do not rely on the LCD brightness alone since that can be off and is adjustable too. Only the histogram can easily tell you if there is over-exposure. You may also have adjusted exposure in the setup menu under exposure adjustments (or something like that, I do not remember the exact name).
The more sophisticated metering modes will statistically give you better results (Matrix on Nikon or Evaluative or Multi-Segment for other brands)because the consider more of the scene. On your Nikon, you can also change the Center-Weighed metering mode to perfrom Average metering which considers the entire scene but much more simply.