Thanks for the feedback everyone! So based on the information here, it seems that a girl is physically attractive enough when:
1. I actually feel physically attraction for her.
2. I have no concern that I'm "settling" in terms of physical appearance. In other words, there is no voice in the back of my mind telling me that she's not good looking enough for me.
This seems right to me, and was the assumption I was going off of so far.
On a related note, the post Franco linked is deeply concerning to me. Mainly because there was a consensus that for the most part, even the higher level guys meet only one or two GF quality girls per year. This is very demoralizing and maybe even a little frightening to me, because this means that no matter how good I get at this, there's always a chance that I'll never actually be with one of these girls. Mainly because as we all know, this is a numbers game. And if I'm drawing from a pool of one or two...well obviously, the numbers aren't exactly in my favor. :/