Hi Paul....
Well....here is my take on what a good histogram would be....
Histogram is basically a guide for the users to have a feel of what the overall
distribution of the photo - if checked through the smalled LCD.....
On the Histogram:
a) Extreme left wall - highlights or white (areas on images categorised in this region are normally unrecoverable....resulting loss of detail....)
b) Extreme right wall - Black or shadow (areas on image that may be too black/dark or in dark shadows that may not be able to be recovered.....through post process....
Therefore....in general....a good histogram would be one which has a distribution centralised spread across the left and right....
If there are many shadows or highlight /white areas, then can't help it.....these would come up as spikes close or on the extreme left or right walls...
So depends on what you are shooting (scene) couple the captured scene with the histogram would give one a better idea on the overall subject brightness range of the scene.
Hope it helps...
rgds,
sulhan