The easiest way to look at coaching separately from talent is to grade it as compared to recruiting classes. Teams "should" perform roughly at the level of their recruiting rankings. So an average coach, all things considered, should have a team playing roughly where its last 5 or so recruiting classes have been ranked.
This is the case for BYU, roughly. Our average recruiting class rankings (from 247 Sports) for the past 6 years are listed below. The average for the past 5 years is 68. And when you look at S&P (which I'm convinced is the best overall computer ranking system for football) BYU is ranked 73.
https://www.espn.com/college-football/story/_/id/27997932/sp+-rankings-week-10-getting-ready-monster-week-11
So we are performing (at present) about 5 spots below our average recruiting ranking for the past five years. That suggests that, relative to our talent, we are playing right about where one would expect us to play. Our coaches at BYU have always needed an Xs and Os advantage to have the success we've come to expect. Right now we don't have that.
Last year, BTW, it appears the coaches were a little above average. Our season ending S&P ranking was 46, while our 5 year recruiting ranking average (at that time, going back to 2014) was 64. We were performing 18 spots better than our recruiting average, which is pretty good.
2014 = 64
2015 = 65
2016 = 49
2017 = 66
2018 = 78
2019 = 81
2015-2019 Average = 68.
2014-2018 Average = 64.