Stock paint: gum solutions
Loris, I mix my stock mixes by eye rather than by quantity, but just eyeballing your amounts I'd agree with Marek that your paint/gum ratios seem on the high side for tricolor. Since the PY 151 is a weak pigment, it's possible 1:2 might not be too far off, though I've not used PY 151 beyond an initial test because it's just too light a yellow for my taste. My ratios for PV 19 and thalo are somewhere between yours and Marek's.
While I would agree with the ranking of amounts for these three pigments (most for PY151, middling for PV19, and least for thalo) you would never find me making general statements about relative amounts of "yellow" "magenta" and "cyan" because pigment strengths don't cluster in hue ranges that way, in other words yellow pigments aren't generally weaker than magenta pigments aren't generally weaker than cyan pigments; they vary all over the place. For different pigment combinations, it could go in different orders.
As Bruce McEvoy describes so competently on that page (which I hadn't found my way to before-- what a wonderful site; there are layers and layers of information to discover) there's a wide variability between pigments and even between brands of pigments as to how much pigment is required to achieve an optimum luminosity/color saturation, and that's why I mix by eye rather than by measuring. I check that the gum/pigment mix is at optimal color saturation for that particular pigment by brushing it on a scrap of paper and comparing to standard swatches that I keep. Depending on the pigment and the use I have in mind, that can be as little as a squeeze of the tube (say for thalo or lamp black) in 20-25 cc gum and as much as a whole tube in 15 ml gum, to make a very dark black with PBk11. Then I either use that mix as it is or dilute it, depending on the purpose I have in mind. For example, for very subtle smooth tonal gradations in highlights and midtones (I'm thinking monochrome here more than tricolor) I dilute it more (and then usually add another printing with a more concentrated mix for the shadows, unless my goal is a very high-key print) than I would for either a one-coat gum or for tricolor. So I use many different pigment concentrations for different purposes.
I don't find it necessary to print more than three layers to achieve a full tonal scale in tricolor, but I also think that printing with more layers of a lighter pigment mix gives a lovely delicate effect, so it really depends on the image and what you want to do with it.