Massimo, your question sounds familiar - I asked it of a few people only a couple of months ago :) 

One possible answer is this: https://groups.io/g/carbon/attachment/13286/0/UltraStable%20Emulsion.pdf
I was directed to this by Kees Brandenburg and Charles Berger kindly posted it in this thread, which touched upon glop recipes: https://groups.io/g/carbon/topic/96375003
That thread is mostly about DAS, but the Ultrastable recipe also gives a starting point in terms of pigment loading.

However....there's no single answer to your question.

One of the people I also conversed with and who was really helpful on this, was Calvin Grier. In autumn last year, we exchanged some emails, and I hope he won't mind if I quote one particular piece of advice he gave me:
"Try to make the best individual emulsions you can and that's it. "
Paraphrasing in my own words, this is what he conveyed to me: in color carbon tissue, what you aim for is good saturation (high chroma), so make color tissues that give the maximum chroma you can manage. You'll find that beyond a certain point, chroma simply doesn't increase anymore as you keep adding pigment. Contrast of the tissue will change, but how relevant that is, depends on your workflow: particularly if you work with continuous tone negatives (like in the book by Sandy et al.) vs. halftone screens (as Calvin does/advocates). 

As long as you get good/high chroma in each of your tissues, you can calibrate/profile your way towards good color accuracy, assuming a hybrid approach where you start with a digital image and output that to color separation negatives. Calvin has a really nice series on calibration for alt. processes, that specifically goes into getting good color reproduction as well: https://thewetprint.com/digital-workshops/ I highly recommend that series of e-books, as it'll save you lots of time exploring directions that turn out to be dead-end streets, or ridiculously complicated. 

Moreover, note that the answer of pigment loads depends a lot on what (source of) pigment you're using. The difference between e.g. powdered pigments (which you disperse yourself), pigment pastes (concentrated, but already dispersed) and watercolor paints, for instance, is huge - they will span a range of more than an order of magnitude. With powdered pigment, you may be working with a 0.1% - 0.2% w/v concentration in the glop for cyan, with pigment pastes this may be around 0.2-0.4% and with watercolor paints it may be up to 2% or more. So even if you find specific numbers somewhere, it'll be hard/impossible to transpose these to your particular pigments - unless you exactly duplicate someone else's workflow and materials.

As to testing different pigment loads: the only feasible method I'm aware of relies on measurement. This requires either a color densitometer or a photospectrometer, although perhaps with a calibrated scanner (or even digital camera and controlled lighting), you can get somewhere as well. I tried a color-calibrated scanner initially, but getting an i1Pro did make things a whole lot easier - and much more accurate as well. Trial and error combined with visual evaluation of single color test prints theoretically should work as well, but I don't think it's a very realistic approach if you're serious about color reproduction.

Full disclosure: my own color carbon project is on the backburner a bit because during testing, I decided that halftone screen negatives are really a superior approach to regular negatives when it comes to linearization and consistency. I also decided that DAS-incorporated tissues are essential for good consistency, so that's one puzzle I had to solve first (and I think I've got that one sorted now). I'm currently contemplating if and how I'll proceed with the halftone negatives, and enjoy monochrome carbon printing from in-camera negatives in the meantime. 

If you're curious, here's a collection of blogs I posted over the past few months dealing with color carbon and the several challenges I ran into: https://tinker.koraks.nl/tag/color-carbon/
Note that initially, I sent myself on a wild-goose chase trying to balance the pigment levels of the color tissues so that with equal exposure, they would result in a neutral grey print. For a couple of reasons, that's not really necessary and actually not the best approach, so save yourself the trouble of doing that. Just one of the many examples I ran into where reading Calvin's books before I started would have saved me a lot of trouble. 

Whichever route you end up choosing, I'd be very interested in reading/hearing about your progress and findings!

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https://tinker.koraks.nl/