Most regions use a point system for seeding. For example, 3B uses:
9 pts for 4A-6A win
8 pts for 1A-3A win
0 pts for 1A-3A loss
1 pt for 4A loss
2 pts for 5A loss
3 pts for 6A loss
Divide your # of points gained, by number of games, you get a rating. Number of regional qualifying teams...