Print process requires that I use a spot color for black. Recolor artwork changes percentages.

Illustrator CS6 (other users on 5)

Mac OS 10.6

I just need to go from K and tones of K to a spot color and tones of a spot color.

I'm using recolor artwork because I can't merge into K. The art comes in from our customers as K so we can't just design differently.

Edit: For clairification. The rectangles on the right are k and tones of k. The rectangles on the left are the results after converting to a spot color with recolor artwork. I honestly am not married to the idea of using recolor artwork. I just need a way to accurately move the tones from k to a spot.