You can never trust the values of your monitor. Your monitor can show values outside the color gamut of your printer. What you need is a pantone to process color book. This will allow you to see the spot color pantone and the CMYK process of the spot color right beside it. In many cases the spot color pantone is very different from the process counterpart. These books also show you the CMYK values so you can see what colors are represented in your spot color.
In my experience it is always better to change the spot colors to CMYK yourself rather than let the printer rips to do it. That way you know the exact value of what you are about to print.
In my experience it is always better to change the spot colors to CMYK yourself rather than let the printer rips to do it. That way you know the exact value of what you are about to print.