Where did you go? You were online when I asked my question. Okay, I'll assume your RGB values are in separate columns (Columns A, B, C for R, G, B respectively) and that your output is to go in Columns D, E, F, G (for C, M, Y, K respectively). Also, I am assuming Row 1 is used for headers. With that said, put these formulas in the indicated cells and copy them down...
Make sure to format the cells in Columns D, E, F, G as Percentage with zero decimal points.