In recent jobs, I have skewed away from pre-mixed rich black formulas in favour of making a black that fits dominant colours in the publication. For example, in a publication with a dominant full magenta colour scheme, I used a pure magenta rich black (C0 M100 Y0 K100) and the results came out great.
In a current job, I have a dominant green that is C50 M0 Y100 K15. To create a fitting rich black for this identity, my first impulse was to add half the cyan and Yellow values to the existing 100% black. That yields C25 M0 Y50 K100. Is that too light?
My question is: how much ink, in percentages, should I add to the 100% to at least have a satisfying rich black? How ‘wet’ should a good rich black be?
Edit: I understand all that is said in What kind of black should I use when designing for CMYK print? and I’m applying most of it. None of the answers to that question specify how much ink a rich black should have at minimum to be a rich black; to avoid the pale, translucent black of 100K.
There is no answer to your question as such: everything above C0M0Y0K100 is darker than process black. However, consider that 100% ink means that your printer fills the entire raster, while 25 fills only a quarter of a raster.
This means that the lower the value of your other colors the more uneven it is, so it might be perceived as grainy. So while M100 K100 is extremely even, C25 M0 Y50 K100 might appear more uneven. How grainy people perceive it is hard to say, since it depends on the medium, print resultion, raster type and distance to viewer. Most likely this is not really an issue.
However if you go for dark BLACK with a hint of undertone, then I would fully saturate the primary color and add black. Then bring it down to a 200-300% coverage range.
But then I have the option to make test prints (and sadly, the duty to clean the machine if I just get the color all over the place). So I would print a few swatches.