Here's another way to look at it.
At the end of the day, the defense's job is to keep the other team from scoring. Some teams do it by limiting yards, some do it by simply limited points.
When an opponent has the ball, there are essentially five things that can happen for a defense:
A punt
A missed FG
Turnover on Downs
A Takeaway
Offense Scores
Against Jackson State, they had 11 possessions. 6 ended in punts (54.5%), with 1 3 and out. 3 ended in a takeaway (27.2%), and 2 ended in an opponent score (9% TD, 9% FG)
If you apply a defensive efficiency formula to the above numbers (similar to passing efficiency for a QB), you could do something like this.
Ends in Punts or TOD *1 + 1.5*3 and Outs + 3* Takeaways - 1.5 * FG - 2 *TD, you'd get a defensive efficiency that looks like this:
54.5+(9*1.5)+(3*27.2)-(1.5*9)-(2*9)= 118.1
Against Auburn, which is our best defensive effort, Auburn had 13 drives. 6 ended in punts (46.1), with 3 3 and outs) 5 ended in takeaways (38.5), 1 ended in FG, and 1 ended in TD (special teams counts as a drive in this formula)
46.1+ (23*1.5)+ (3*38.5) - (7.6*1.5) - (7.6*2) = 169.5 Outstanding
Lets take a look at Alabama shutting down Western Kentucky using this as a formula.
WKU had 10 drives. 6 ended in punts (60%), with no 3 and outs, 4 ended in takeaways (40%). No scores. A dominating defensive performance.
60.0 + (3 *40) = 180.0.