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.
Of course I do not get the colour I want.
If you get 0 % C, 48 % M, 74 % Y and 9 % K, it is (except the small rounding difference) the same color (and also the same as 9.75 % C, 52.5 % M, 76.25 % Y and 0 % K).
5 % C, 50 % M, 75 % Y and 5 % K: R = (1 - 0.05) * (1 - 0.05) * 256 = 231.04 > 0xe7 G = (1 - 0.50) * (1 - 0.05) * 256 = 121.6 > 0x79 B = (1 - 0.75) * (1 - 0.05) * 256 = 60.8 > 0x3c
1 - ((1 - 0.05) * (1 - 0.05)) = 0.0975 1 - ((1 - 0.50) * (1 - 0.05)) = 0.525 1 - ((1 - 0.75) * (1 - 0.05)) = 0.7625
9.75 % C, 52.5 % M, 76.25 % Y and 0 % K: R = (1 - 0.0975) * (1 - 0.00) * 256 = 231,04 > 0xe7 G = (1 - 0.525) * (1 - 0.00) * 256 = 121,6 > 0x79 B = (1 - 0.7625) * (1 - 0.00) * 256 = 60,8 > 0x3c
1 - ((1 - 0.0975) / (1 - 0.0975)) = 0 1 - ((1 - 0.525) / (1 - 0.0975)) = 0.47368... 1 - ((1 - 0.7625) / (1 - 0.0975)) = 0.73684...
0 % C, 48 % M, 74 % Y and 9 % K: R = (1 - 0.00) * (1 - 0.09) * 256 = 232.96 > 0xe8 G = (1 - 0.48) * (1 - 0.09) * 256 = 121.1392 > 0x79 B = (1 - 0.74) * (1 - 0.09) * 256 = 60.5696 > 0x3c
P.A.