I'd say
80-85 works typically well for all skill positions on offense & defense. That way, they don't stay in too long, nor sit too long either.
But Think of it like this...At the beginning of each game, every player starts out at 100% energy. Then as your game plays on, the players get reps, get tired & subbed out. They sit for a breather, which replenishes their energy back to 100% and get subbed back into the game...
...But just like in real life, in NCAA FB all the players begin to show longterm fatigue and then only start recovering fully at 95% energy, then 90%, 85%, 80% etc... So if you set your Sub-In at 90 or higher, then you'll start to see the "disappearing RB" that never gets subbed back into the game in the 4th qtr. That's b/c he now only fully replenishes at 85%.
So for me, I felt that
Formation-Subbing represents a true to life way of getting some guys reps, whom I
want to see play.
Auto-Subbing for me represents a universal
Stamina Slider, and the pace in which players realistically sit for rest. After 3-4 carries, my starting RB is almost guaranteed to sit for a breather. Typically I'll see my 2nd string RBs get more carries in the 3rd & 4th qtrs. Ending up w/ around 6-9 carries. So I balance it out w/ my Formation Sub packages.
An example of how my RB usually looks w/ 20-25 carries:
1st Qtr: 100%
2nd Qtr: 95%
3rd Qtr: 90%
4th Qtr: 85%
If you want to see 5th & 6th string WRs get reps, I would suggest having a few 5-wide formations, and possibly a couple formation subs.
The auto-sub won't work perfectly, nor the same for every game b/c of the amount of reps a player gets & how they perform.
sorry so long-winded...