If I recall correctly, the peak of the intensity distribution should be
at least 10% of whatever the saturation level is of the camera one is
using. Is this correct?

If I also recall correctly, when taking a flat field, the intensity of
the image (which is hard to define - do we look at a bright area on the
field or what?) should be at least 30% of the saturation level of the
camera one is using. Is this correct?

I know these are not hard rules since I have violated the first and was
able to get usable results. Maybe this says it is probably also a
function of the signal/noise ratio.
Stan