--- In colortheory@..., "Mike Abbott" <mda@...> wrote:

I'm wondering if anyone can point me to reliable information / guidance on
the use of UCA. I'd like to understand how and when to apply it in the
context of using Photoshop's custom CMYK dialog.

Hi Mike,
in addition to what Dan said, the only case in which I've used UCA is when I had Black & White images that need to be converted in CMYK.

As Heavy GCR features CMY values still higher than the Black in the highlights, I usually do a conversion with Maximum GCR with a slight amount of UCA.
This gives me an image in which the Black is always higher than the CMY channels while increasing the weight in the three quarter tone tones and the shadows.
As for the value, it depends from the image as UCA works mostly in the shadows.

I would say not more than 20-30% as I would not have any of the CMY channels to be higher than 40-45% maximum.

Of course the final result must be evaluated with a proof to be sure that everything is ok.

Hope this helps,
Alessandro Bernardi