If 24 is not a good metric the right metric has to be something similar. I think BYU's number of possessions compared to the average number of possessions is super relevant and you do a great job bringing that in. If BYU is at the average and other offenses are scoring at or below average then the offense has to score more.
Our defense definitely needs work but I have to say the offense frequently gets a pass which is really ironic given that the offense is the more talented group. The outcome of the Oregon game for example is attributed by many to the Defense but our nationally applauded Offense playing so poorly has to be a major factor. It is very rare for us to lose a game if we score above 24 so we have to ask if its simply mostly the Defense's fault.