First, I am not familar with dual K inkset described by Paul Roark. But you mention that you printed at 50% limits on HPR 308, which is a coated MK paper, and as-such should yield Lmin of approximately 17.7 to 16 (Dmax ~ 1.6 to 1.7) using single MK. So you may want to look into, perhaps increase ink limit to >50% or use "Black Boost" ~70%
Please refer to picture below on how to set the density of lighter shade of grey inks. Mind you this is only for 3 shades of grey (K, LK, and LLK) as an example. Toner inks utilize the density numbers also, but rely on "Limit" to determine how much of the toner gets used. You that either visually, or ideally with a spectro.
Say at the calibration ink density, the solid blue line represents density of K in 5% step file. Solid orange line represnts LK ink, and solid green line is LLK ink. What you want to do is to find Lmin of LK and LLK (dashed orange and green lines respectively), and find the patch of K which prints at the same density. In the case of LK (orange vertical dash) gives you ~25%, and LLK (green vertical dash) gives you ~15%. These are the numbers you input into "Density." If I undersand your graph, it seems you are looking at these numbers from succesive lower density inks. That would not be correct. All densities should be relative to K.
