>We have a client that wants to match a RGB value to a spot color.
Unfortunately you have not posted where they want to use the resulting RGB color, the wording of your question leads me to think that you're looking for an RGB result for a spot color. Either way my last suggestion will give you the best spot color choice from an RGB starting point. For example many spot colors can be out of the sRGB gamut so an accurate web representation is sometimes not really possible.
The factors to be considered before you choose a spot color to RGB conversion method are these.
1. Are you useng the resulting RGB for placement and viewing only on a display, web or presentation?
2. Are you using the resulting RGB for Digital Print?
3 Are you sending the converted RGB file for editing to other applications and if so to which ones?
The guiding parameters are the Pantone specifications which at this point are for using the LAB color space for converting spot color to RGB, unfortunately Adobe has not kept up so the Adobe conversions will not be the same as applications that support the proper Pantone process.
However if editing by others is your goal then use the application that will be used as the destination and for the most part many will use Adobe and default Illustrator tables will work. However understand that Adobe has fallen behind on compliance with Pantone specifications and Illustrators conversion of black when rasterizing is completely NON-ICC compliant.
With that said if web, presentation or digital print is your destination and more accurate conversions are your goal then I suggest using Windows Vista or Windows 7 and the WCS color engine, CorelDRAW X5 uses the current LAB specification and with a good media profile (meaning a good gray balance) has been giving me the best result in print or web. This will aslo give you the most accurate spot color from an RGB source.
Many spot colors will not convert properly to print, more convert to RGB properly and will produce a more satisfactory result in print if converted to RGB in the application then printed digitally, rather then if printed as spot from the application to the media profile.