There aren't many "SOS" calculations that don't include garbage like recruiting classes, who was lost/gained etc, but here's one. It's straightforward and calculates the team's difficulty based on their opponents and who they played and how they faired in those contests.
Strength of Schedule Played and Strength of Schedule Remaining for all 133 NCAA Division 1 Football teams.
powerrankingsguru.com
Yeah, it uses their power rankings as a point of reference, so not perfect, but it cuts out a lot of that other garbage. I'll never understand the "stronger conference" stuff since most don't even play all the teams in the conference. The schedule is what matters. You'll notice in this ranking that the point spread between 1 and 50 is 14 points roughly. There isn't a huge gap like one might think when it comes to "SOS" if you take a bunch of the subjective nonsense away. Even ESPN's "strength of record" calculation has the two incredibly similar. I have no idea what goes into their numbers either. I just know that's what they allegedly use.