I’m following these instructions here: Profiling a camera with darktable-chart
Also, this can be useful: Patch color definitions for Datacolor SpyderCheckr 48 - Christoph Bartneck, Ph.D.
They’re the lab values for the color checker.
Hope this helps!
EDIT: unfortunately, there seems to be another problem. The size of that .cht grid is wrong.
I can’t have the grid match the size of the boxes. I’ve tried with different images , I’ve corrected the perspective, but the problem seems to be caused by the .cht file itself. The size of the hinge seems to be recorded wrongly. I’ll read the .cht file documentation and try to understand how the measurement is expressed in the .cht syntax, hopefully I’ll be succesful…
EDIT 3: it was a silly mistake of mine. Here’s the final .cht code, which works and aligns perfectly: