That is a great question.
So let me take you through my thought process and what I do. I will start with David Johnson.
Stud 3 down back (rare in todays NFL) and they have Chris Johnson and Andre Ellington behind him.
I will have Chris Johnson in on Let's say either the I Formation tight (for his one set where he get's some running plays) and Ellington in on a few shotgun formations and a 5 wide formation as a WR.
That's it.
But let's take a team like the Bengals with Hill and Giovanni Bernard.
Hill on all big set's, jump set's and goalline set's. Gio in on all single back 3 WR set's, I formation 3 wide, and most shotgun sets (not all but most)
So that gives you an idea with those type of teams.
Then when you have a true RBBC I will split up the formation subs pretty evenly. And use the better pass catcher as the 3rd down back and passing down back.