D – Class: 1 to 999 Battle Power
C – Class: 1,000 to 9,999 Battle Power
B – Class: 10,000 to 999,999 Battle Power
A – Class: 1,000,000 to 9,999,999 Battle Power
S – Class: 10,000,000 to 99,999,999 Battle Power
Beyond S – Class: 100,000,000 and Beyond Battle Power
Note: A difference of 1,000 is surmountable by a charged attack or finding an opponent's weakness as shown by Goku and Piccolo in the Raditz arc of Dragon Ball Z.
Also, another major note is that everything from DxD and Fairy Tail has been scaled up to keep up with the early sagas of Dragon Ball Z and eventually Super. This is because I don't want their training to be obsolete and rely on ass pulls like Roshi, Tien, and Krillin in the Cell, Majin Buu, and TOP arcs.