It's common in my industry for reference/archival photographs to be taken with a combined scale, grey, and colour checker card (often with rectification targets too). In most cases, the standard is a four-panel colour checker. A common card people use is this, or something like it. 
I have been reviewing a collection of unprocessed RAW photos at my current company, and they have cards like this. I've discovered, however, that no one at the company knows how to process RAW photos, and they just use the camera JPEG outputs, so I am working on processing the backlog of RAW photos. I am used to using a 24 or 48 colour checker, but have not used four-colour checkers before, and I cannot find any information on using them for colour correction.
In particular, I need to know how to make use of these in Darktable (v4.2.1). I have the CMYK values of the cards used, and have converted them to the RGB and hex values. But I cannot work out a way to calibrate the colour using these cards other than by eye.
Can anyone suggest a workflow for this scenario?
