Digital BW, The Print

I don't have Photoshop curves posted for the option of having UT-FS-Y in the
yellow spot, but in general, I have the Blue curve (controlling the yellow
ink spot -- FS-Y) carry the image entirely until about 229 on the horizontal
axis, where I start the other inks (Red and Green curves).  

As an example, for Kirkland, a neutral curves set I use has the following
points from 0% (255) to 25% (191):  (255, 254), (251, 187), (247, 168),
(242, 147), (229, 99), (216, 88), (204, 83), and (192, 82).  

For glossy Kirkland, I taper it off to (127, 146), (25, 0).  For a matte
paper the blue curve has to be full on at (0, 0) to help turn on the black
ink.  So, for a matte paper it'd be either raised and then lowered again or
go about level, perhaps at a bit higher point so that the other inks were
not limited too much.

The Red and Green curves, as noted, start at 229, and are raised about 5 at
191 and about 15 at 127 relative to the starting curves that did not use
FS-Y.  I don't have a good comparison here, because I used the Ilford curves
for Kirkland, which were not right on to start with.  Print a 21-step with
the current, starting curves and just eyeball the 50% and 25% patch
densities to see how much the Red and Green curves need to be raised to
offset the addition of the FS-Y.

Hope this helps.

Paul
www.PaulRoark.com