Pavel A. da Mek wrote:
I can not enter colours like 5 % C, 50 % M, 75 % and 5 % K, the number always change upon entering the fourth value such that one value is 0 %.
This is the purpose for what the fourth (black) coordinate (ink) is used: to use an additional black filter and only two of the three color filters.
Maybe theoretically, but not in practice. In the prepress world is common to use the four values of CMYK, for example in 'rich black' (http://marvin.mrtoads.com/richblack_vs_plainblack.html) or in PANTONE to CMYK transformations. The algorithm you use is ok to convert RGB->CMYK or CMYK->RGB (without color management), and show the result in a screen, but looks different if you print it. I think inkscape only translates CMYK values to RGB values (and later from RGB to CMYK in the color palette, lossing the original CMYK colors) but really don't support the CMYK color mode.