First off, eliminate custom mixes wherever you can. Develop a set of standard colors and offer those. Most will just go for what is available.
Next, charge more for WB and HSA mixes. It's +$10 over regular "non-standard ink color" mixes here. Still probably not enough, it can soak up hours of a week, even for those skilled in color mixing. Charge whatever you think is fair and also use the charge as a deterrent so folks pick a standard color instead.
These inks will simply need adjustments from formula mixes. Accept it and then do everything you can to make it easy to make those inevitable adjustment's- use the ink system's software interface thingy, have a template in your job sheet (you got one) that auto totals percentages as you add inks or put out a marker board to jot down mix numbers, whatever it is, every little thing helps.
At some point your ink person will get some intuition as to what will shift which way and begin compensating for it on the first mix. For instance, we instruct our ink girl to omit black and white pig and then add back in stages. After some time she can guess how much of the white and black in the formula is actually going to hit the color. TonyP always alludes to this and it's true- the mfg formulas are not accurate and often extrapolated, overusing white and black where they are not appropriate. Also, you may hit a wall with certain systems and certain hues, be ready to accept that to and always warn your clients that you can't actually produce a match, just something that looks good and is close.
Hope that helps.
