Even 1% will have a noticeable effect, if 2% is too much.
>
> I'm using the 'tranfer function' function, good suggestion on looking at
> the histogram.
That's when you finalize the image for output. I'm talking about your
photoshop manipulations to make a pleasing image from your scan. Always
use "curves" or "levels" to adjust contrast at this stage, and try to
have as solid as possible a histogram, along with a satisfying on-screen
image, before you apply the transfer function.
>
> Right now I'm working with portraits, skin tones should work pretty well
> as a standard to judge against. I'll check out Davids work as well.
>
Skin tones should be a good test indeed.---Carl