2015-08-12 by richard@...
The gray highlight and gray shadow settings both have a default setting of 4 (if left blank), and the maximum value of 10,000. In reality, settings higher than 20 will result in barely noticeable changes, and most of the lightening affect happens with settings between 6-15. It seems that the highlight setting lightens the tones between 0 to about 25%, and the shadow setting lightens about 75% to 100%.
I've made illustration for the section in my book that covers each input in the ink descriptr file and shows the degree of change in the curve based on different settings. There is nothing that suggest that a setting higher than 50 will do anything beneficial for the highlights/shadows, and it is better to use a gamma setting or gray curve correction to have more dramatic changes to the shape of the curve.
With the Epson inks a highlight setting of about 6 seems a good place to start. Depending on your ink limits and paper type a shadow setting of about 15-20 can be good for keeping the shadows open enough to get through the linearization step without error.
These settings are useful starting places for multiple gray 3rd party inks too, but since there are many other factors to consider when making profiles for those ink sets, these are only starting points. If you are finding that your print is printing too dark even with those settings, you may want to try increasing the gamma slightly—maybe 1.05-1.3. Make small changes, because a little goes a long way there.
Hope that helps,
Richard Boutwell