I have a client's color palette document with RGB values listed, but they go to 255/255/255.
My pitstop RGB color values only go to percentages of 100. Having a heck of a time figuring out how to convert the 255 values to percentages of 100.
Unfortunately, I don't have any documents that I can just grab the color from. I need to manually input these values into the action list, but converting them isn't coming easy.
I've been looking online for a converter but perhaps I'm not using the right search terms. Coming up empty.
RGB to RGB (values!)
In your PitStop Preferences go to "Units and Guides" and switch off "Show percentages for RGB colors" 
I don't know from which version this was introduced. All I know is, that it's there in both 12 and 13, so you'll probably need to upgrade to get this feature.
You can go from 255 RGB values to percentages like this:
multiply by 100, then divide by 255
For example RGB of 51/102/204 (a relatively dark blue) would be:
51 * 100 / 255 = 5100 / 255 = 20% Red
102 * 100 / 255 = 10200 / 255 = 40% Green
204 * 100 / 255 = 20400 / 255 = 80% Blue.
The other way around is multiplying by 255 then dividing by 100:
20% Red * 255 / 100 = 5100 / 100 = 51 Red
etc.
multiply by 100, then divide by 255
For example RGB of 51/102/204 (a relatively dark blue) would be:
51 * 100 / 255 = 5100 / 255 = 20% Red
102 * 100 / 255 = 10200 / 255 = 40% Green
204 * 100 / 255 = 20400 / 255 = 80% Blue.
The other way around is multiplying by 255 then dividing by 100:
20% Red * 255 / 100 = 5100 / 100 = 51 Red
etc.