I usually use the colour bridge coated guide to get a match between my cmyk and spot colour. The problem is I find the RGB colour match in the bridge book (which im aware is a conversion from the pantone coated values) is usually way off my cmyk match. What I tend to do is use a programme conversion from cmyk to RGB to get the closest match. Is this bad practice or should I stick to the colour bridge RGB values?
There is one real answer and several applications/implications.
The answer is to define a standard. This means that you need to define the profiles used on each color mode (which is not the same as color space).
The applications. Now we need to apply it.
On some tests I did some time ago I have the impression that Pantone, as it is an American standard is using... American standards... More specifically they are (or were) using Adobe 1998 as RGB color space, and SWOP v2 as the CMYK profile.
If you want a generic color "equivalent" that people can easily reference, simply use either the Color Bridge data or use the Pantone's website values.
It is really weird that Pantone does not define this. My guess is that they want people to think of them as the "absolute color guide" and avoid any variable on the mix... which is an incomplete workflow. Simple people do not care about it, and "pro" users know some workarounds.
But if you are using some other configuration, as a European standard, some Fogra color profile, and the files are exported directly from a specific software, like Illustrator, Corel Draw, etc. probably the best option is to use the software using spot colors, and then measure the colors using a CMYK color picker.
For the RGB conversion, I would directly export a file to PNG, embed the color profile, view them using a web browser, and measure the values using an external color picker measuring it the colors on the web browser. This way the exported files will match any hexadecimal code you throw at any web design.
One problem is that if you are using Adobe1998 as an RGB profile if for some reason it gets dropped, the file will be "assigned" sRGB, so you probably want to do that as the configuration on the initial software.