Not 100% sure but here's my best guess:
FY2016 - 36 Colonels selected for promotion to Brigadier General
FY2016 - 3,472 Colonels in the USAF
In 2015, there were a total of 3,865 Lt Cols eligible for promotion out of a total of 10,215. That is 37.8% of all Lt Cols were eligible for promotion. Let's assume that the same percentage of Cols are eligible for promotion. In other words, what is 37.8% of all the Cols in the AF in 2015. That would make 1,313 Cols eligible for promotion to Brig Gen. The selection rate of 36 Brig Gen selects out of 1,313 Cols is 2.74%.
Final Guess: 2.74% of O-6 get promoted to O-7