I've been using the following formula to track Pts per Possession since 3.0 was launched:
Pts / (FGAt + 0.44 × FTAt – OREBt + TO) will equal points per possession
Pts = Points allowed
FGAt = Opponent Total FGA
FTAt = Opponent Total FTA
OREBt = Opponent Total Offensive Rebounds
TO = Opponent Turnovers
As of today, the breakdown using the above for the boundaries right now are:
| 81.68% = Very Good |
| 82.03% = Good |
| 95.126% = Good |
| 95.14% = Neutral |
| 108.82% = Neutral |
| 114.32% = Bad |
| 126.15% = Bad |
| ? = Very Bad |
Sorry never had a Very Bad defense rating yet so I can't say how much worse you have to be in order to get that.