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I'm pretty sure if you have software like illustrator, photoshop, or even corelDraw you can convert the RGB color to pantone. It should match it up pretty well.
Pantones are defined in the Lab colour space, RGB values are converted to Lab according to your icc profile. So the answer is 'it depends which icc profile you have'.
Photoshop is coming up with 1797 C but it's definitely a lighter red (1797 C = RGB 201/40/45).
John's 187 C from Corel is darker by a similar amount (187 C = RGB 172/26/47).
If you mixed 1797 and 187 together or averaged their mix ratios you'd probably be very close.
Not exactly.... There are too many variables involved for it to be that simple. He is basing the desired color off the monitor he is looking at. His monitor could be calibrated to see that color as a maroon. If he wants a spot color, he will have to check out a PMS book.
Agree without printing on the same printer and media you will not know for sure. If you have eveything syncd with a custom color profile. You have a chance.
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If color is super important, the only way to do an accurate match is to do a sample print of the color then match the printed color to a PMS chart, as David and others have suggested.
No use trying to match from an uncalibrated screen in a mixed light environment as that will make it look different as well.
What looks good to me may look something else to you.
With so many variables, it makes matching a physical sample the best way to go.
For anyone who is serious about the print business (garment or otherwise), a Pantone book is a small price to pay for confidence in the end result.
With one, you, your customer and your printer are all on the same page when it comes to what color to use and how the finished color will appear when printed.
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But in the end, it is subjective as that rgb color is not in the Pantone book. Also, if you print color charts on a printer that is not calibrated, you are no better off than picking off your monitor. I work with $50k+ printers that require calibration twice per day and they still sometimes need manual tweaking to print exact Pantone colors.
Pantone color 
