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I would mix according to package instructions, although in a much smaller batch, and then try it. Since you're going for black, if it's too light you can add a little more dye and try again.
I followed the package instructions 40 years ago when dying sawdust green for grass, before the days when I could afford ground foam.
In the January 2016 Model Railroad Hobbyist magazine (free on line) they suggest using various sands for ballast (sized by scale and type of ballast). They put the sand into a plastic bag and then add liquid RIT dye straight into the bag. Depending on the quantity they use a few drops to get the colors they want.
To continue my ballast questions I am still unsure of what color would be best for my ate steam era (late 40s ) that I still remember, and I seem to remember the ballast was mostly black/cinders. But that seems to be hard to make, and I am WAY to tight to buy ready made. Any more suggestions?
Besides use brown walnut shells or. chick grit, if I can find it