Color convertion

Numbers are color space specific!

You need to specify which RGB color space to whick CMYK color space. If you don't specify, numbers are undefined and meaningless. sRGB and Adobe RGB won't produce the same numbers. US Web Coated (SWOP) and Coated FOGRA39 won't produce the same numbers. And so on.

The internet is full of these "conversion tables" and they are all completely useless, for this precise reason. They don't specify a color space.

In Photoshop, conversions assume whatever you happen to have set as working spaces. Those are the numbers given.

For RGB, you generally have the choice between some standard color spaces, sRGB, Adobe RGB, ProPhoto, DCI-P3. For CMYK it's a lot more complicated. A CMYK profile describes a certain print process - an offset press calibrated to a certain standard, using certain inks on certain paper stock. These standards vary in different regions. The only thing you can be fairly certain of, is that the Photoshop default, US Web Coated (SWOP), most likely doesn't apply.

In other words, there is no such thing as "RGB" or "CMYK". You need to know which one!

Photoshop's whole architecture revolves around using color spaces correctly. You can't ignore it, if you do, you'll get in trouble.