humm... hi there! =) If i can say my word... well... first.. never work in cmyk... CMYK is not a good profile to work with,... better RGB or LAB. There is not a unique technique to obtain the perfect result. Depends. Sometimes working with the lightness profile under LAB leads to good results, sometimes this is completely unuseful since it can lead to imperfect images. The Lightness under LAB gives only the luminance of the single colors of the image and sometimes does not contains the differences and the characteristics of the colours themselves. This kind of information could be kept if you try to convert a colour image to B/W. For me the best way to proceed could be to observe the various results from Lightness (in LAB) and from Greyscale conversion+some channel mixing and meld 'em together.