I find watercolor paint in tubes to be a very easy material to work with. Someday I will have to try the various inks. It has been 16 or so years since I have used Sumi ink...first bottle was great, the second bottle clumped and gave grainy images so I stopped using it.
I use it by the gram -- the liquid ink can probably be used by volume, but by weight is easy if one has a decent scale.
The watercolors are nice because the pigments are already nicely ground and evenly dispersed in gum arabic. I measure out the amount I need in the plastic 35mm film containers, then add about 20ml of warm water, put on the lid and give it a good shake. I leave it for awhile (while the gelatin is being prepared) and by the time I need to pour it into the gelatin, it is nicely mixed with the water. Sometimes I will add a little alcohol to the container to get rid of bubbles from the shaking before I add it to the gelatin. Watercolors are also nice for slight adjustments of color. I sometimes add a little Burnt Sienna to the lampblack to warm it up a little.