I've never had a litmus test, personally. It's something that's troubled me throughout the years as I noticed I only gauged my thinking through those who can be immediately identified as black. I cannot think as a mixed race person because I'm not a mixed raced person so those are thoughts that trouble me.
I don't personally want to exclude any black person in the progression of a people I identify with on a level that nothing else compares. At the end of the day, I don't know. We could just go by my earlier posts that at some point, it just doesn't matter if you had one relative who isn't of the race you are, you aren't that anymore.