Digital BW, The Print

I am now the proud owner of an x-rite 810 densitormeter and am working 
at finessing curves for the papers and quadtone inks I use in my 3000. 
 I have had good success creating curves 'by the seat of my pants' but 
this approach isn't very efficient.  I'm using IJM/OPM but most of the 
questions I have are generic.  

1. Are there more systematic approaches to partitioning these quadtone 
inks than than the recursive print and twiddle that I use now?

2.  Is there a way of pre-determining what the density of a mix of two 
inks will produce?  More generally how do the densities of two inks 
mix.   Does 50% light grey and 50% dark grey lighter or darker than 
50% dark grey.  I suppose the answer is "it depend" but what does it 
depend on?

3. There seems to be more than one set of curves that will produce 
good results (sometimes the light grey is used through almost the 
entire range and sometimes not).  Are there standard approcaches to 
designing curves?

4. Roy seems to use some voodoo or other black magic to create the QTR 
curves mathematically from the relative densities of the inks.  Are 
there set approaches to this or is Roy just extra clever :)

5. In IJC the curve peak can be increased or decreased which I assume 
is setting the amount of ink to a higher or lower level than the ink 
limits that are set earlier in the process.  Is thre a general rule 
why one would adjust the ink limit in preference to adjusting the 
shape of the curve?

6. Is there any recommended reading material? 

As I said I now have a set of curves that work well for the papers and 
ink I use (thank goodness for the linearization function) but in the 
process I now have a bunch of questions I'd like to sort out before 
tackling another printer/ink combination.

Your help, as always, is greatly appreciated.