Meaning in life. It's subjective. It varies as greatly as there are combinations in infinite sets. Well, theoretically. In practice, I doubt that it does. If we graph it the way we do tag clouds, we would be able to see clearly how common some answers are (eg. God, family, to contribute to "society" or a "greater good" or some other similarly-vague-something-or-other).
I'm probably not alone in not having defined mine. In fact, I'd bet I'm in the majority. It's just that the decision always seemed arbitrary, and those who had arrived at a meaning did so by circumstance. And for an atheist, how else could I have arrived at one? It was certainly not ordained by a God. If I had one, it would have been predetermined by my circumstances, and for some reason besides that it was destined, and in spite of this, it would be able to bring focus and happiness to my life with the one condition of my blind compliance... (Accordingly, I'm quite content without one.)
So, I do the only thing I can do: I let ideology guide me.