The limits for the toner inks are based somewhat on the color of the paper base and some trial and error. I found a good starting point for a neutral tone is about 3-5 cyan and 4-6 magenta, and then blend it in with the curve mixers in the QTR driver.
The addition of magenta is to balance out some of the blue-green in the cyan/gray inks. There is very little extra density coming from the color inks to really change the overall gray curve, but you might want to take off the linearize= line and then do another 21 step measurement to linearize.
One way I approach this is to get the gray curve set with the spreadsheet correction curve template, and then add the toner inks and then linearize the final profile.
I am working on a method of being able to define a set of lab a and b values and a kind of reverse correction curve that will take into account the color of the paper base and the color of the gray inks, and then adjust the color accordingly and have it be precisely applied to be consistent across the whole tonal scale. I haven't been able to sit done and hash it all out yet, and It will take some spreadsheet trickery, but it *should* work.
Hope that helps,
RB