I knew on the day that I pre-ordered that my early access would be based on when I put in my pre order code, this has not changed since July, when they started allowing people to pre order. Me and my friends all pre ordered within about an hour of each other on july 21st and were all in the first wave, but most of us worked today, so we're all spread out as far as levels, though it will probably level out eventually, or not, either way eventually we'll hit cap and play together.
Also, they've explained why they're doing waves of staggered access, they've also explained why they're not letting people know when they're going to be in until they're actually in.
If you don't feel like reading any of the information on why, I'll tell you.
IF BW states that they're pretty sure that you'll be able to get into the game on Wed morning, and something happens, and you don't get in until wed afternoon, or even thursday morning, (or based on some people's complaints, there are people who would be upset if they got in tuesday afternoon instead of wed morning) you are now upset, so they've avoided that, because they aren't telling people ahead of time, they have tempered people's expectations and allowed themselves flexibility to adapt to the situation.