Your best bet is to go a few levels lighter than what your intended result will be.

Reason being, alot of box colors will darken the longer you leave it on. You want a brown result, but if you leave it on for any length of time over the recommended minutes, you will get something closer to black. So keep that in mind. Time is very important. You're better off having it too light and having to color it again than having it turn out too dark.