Also note that display color gamut varies based upon individual monitors (even those that are the same make and model) and graphics display card...  Then heap on top of that the variation that can occur when sent to the printer... Yikes! This isn't exactly an easy task.

Best bet is to try getting some sample proofs made. Then you can adjust your CMYK levels to what seems correct. And run another proof to see if it's right. Keep in mind the lighting of where the final print is to be used, that can also shift colors. (sheesh) Repeat doing proofs until ready for print (or until the budget/printer/deadline says "enough already!") Then you can be sure that the color representation is correct (or as close as you're going to get) between the additive colors on the monitor and the subtractive or translucent colors used in print.

If you can't get proofs or are trying to make a one-off item, next best bet is to use a color-match system such as Pantone. The comparison swatches aren't exactly cheap brand-new, but some people resell old swatch cards online. There might be a little wear and tear, but as long as they're stored away from strong direct light or anything else that could damage them, they'll be up to the job. Then again this may only be useful if your printer supports the system too.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.