The most difficult part of the gum bichromate process for beginners, in my observation, is determining how much pigment to use. Unfortunately, this is not something that can be set out in instructions, or standardized quantitatively, because it varies from pigment to pigment and even from one manufacturer to another of the same pigment. The number one mistake beginners make, in my observation, is to fail to understand the variability of pigment strength and to unthinkingly mix all pigments in the same proportion to gum. If you do that, you'll not have consistency of color saturation across pigments.

I premix gum and watercolor paint in film canisters just to the point where the color is as saturated as it can get, which can't be standardized quantitatively because of the wide variability in pigments and paint brands aforementioned. If you use the same amount of pigment, no matter what pigment or paint brand, you'll not have consistency of color saturation across pigments. I standardize visually by mixing by eye to color saturation, matching the color to previous swatches of the same pigment mixed to printing strength.

I then use that stock mix in different ways depending on the kind of print I'm making and what my aesthetic goals are. For one-coat prints where I want the darkest value possible from the pigment, or for the darkest layer of a multiple print where I want the darkest possible DMax, I use the stock solution as mixed, because the stock solution gives the darkest possible value possible from that pigment. If I want to make a one-coat gum that gives the greatest tonal range possible, I dilute the mix somewhat (how much depends on the pigment). For tricolors, I dilute the stock solution by about half, because tricolors with each layer printed fairly dark would result in a tricolor that's much too dark, the layers being additive. For the highlight layer of a multiple print, or for work where my goal is a very high-key print with no dark values, I may dilute the stock solution by 4 or 5 times.

For each pigment you choose, if you spend some time working with it, you will find a range of pigment concentrations from which you can choose a pigment concentration from dark to light for each particular effect you're interested in achieving from that pigment (within the value range the particular pigment is capable of, of course) .