Nope- teams don't start back just because they are teams. If your number is 20123, you'll be in A corral. (just cut off the 2 and pretend it's not there)
If you are 25123, then you are farther back in the corrals, but not in the way back. Probably B or C is a guess. 29123 would be farther back. Make sense?
I've seen a lot of guys with 4000 numbers, and thought they made the guys start in C corral. If that's the case, then we at least know that C corral will be numbers starting in the 4000s (they do not let guys start in A- and I think B as well)
Reep- I took a look and yep, I think you nailed it! Or really close anyway!