I always thought that all 12 teams in the same conference should have the same prestige.
The Big Six would have A or A- baseline... so Duke and Virginia Tech would have the same baseline.
The Midmajors would have B baseline... A-10, C-USA, Mountain West, WCC, Horizon.
The Midrangers would have C baseline... Missouri Valley, Colonial, maybe the Ivy League too.
The Dreg conferences would have D baseline... 14 of them.