Take precisely 35 drops of black india ink. Dump in some alcohol until it feels right. :-)
I'm joking, but honestly, this isn't a precise science. It is best to start with a fairly weak mixture and just keep reapplying until you get what you are looking for.
Lots of people say to use high percentage alcohol, which has less water. I am not convinced that it is all that important, but I do it anyway since it doesn't cost much extra.
Also, note that artists ink can be purchased in many colors in addition to black. This is very useful. For example, a mixture of black and brown produces an excellent simulation of creosote-treated wood.