There is no "I" in "You"

[Ben Davis]

 I could not have been born in another time because my brain only exists in the temporal 'now' and not at any other point in existence.

This does not necessarily follow.

What?! You're questioning the existence of time? Are you mad or just being obtuse for the sake of argument?
 

Prior to the established "I" what (mundane factors) determines the particular "I" to which you refer?

I've already answered that: the brain that generates 'me' as qualia.

Is it contingent upon specific factors or simply chance?

Both. Among the specific factors are the existence of parents that can procreate a brain capable of generating a person, the environment to support development of that brain to the point where the person becomes self-aware, the absence of destructive mechanisms that could prevent the brain from generating a person, and so on. Among the random chance factors are the specific sperm and egg that came together, the birth location, not being dropped on the head as a baby, and suchlike.

If the latter, under what conditions (milieu) make possible the calculation of said "chances"?

Absolutely no idea. Pre-birth, I'm sure the odds are astronomical, post-birth, the odds are 1-1.

Or alternatively, Can the atheist rationally avoid the inquiry by simply asserting that there was nothing "prior to the established "I""?

There is nothing 'prior to the established 'I''. The existence of the brain that generates 'me' determines 'my' existence.