I don't know that there is a hard and fast rule as to mixing ratios, as it may well depend on the exact color you're trying to get. I've got a project planned for later this year that will involve having to tint clear grain filler drak blue, I recommend what I'm planning to do: experiment with small amounts of grain filler or lacquer and test on scrap wood until you work out the ratio that gets you to the color you want. Use a syringe w/o needle to measure out a precise amount of the grain filler, see how many drops of the tint it takes to get to the desired color; then extrapolate that ratio for the larger amount to mix enough for the project.
