I don't think the breakdown of carries is NEARLY as important as the three factors, in order of importance:
(1) Who gets a majority of general carries. If its 70-30, that would be my rough cutoff, though depending upon the other to, 65-35 would be fine if
(2) The RB that gets 60%+ of the carries ALSO gets all/almost all goaline looks, on a team that runs in the five often
You could also bring in third downs to a degree, because often you get some longer gains via pass receptions, or a 3rd and 20 with a nice easy 12 yard run to help pad stats.
If you are getting 80% of the carries but never sniff the goaline, I call that a committee. If you get 60% of the carries and get ALL the goaline, then that is somewhat of a committee, but not a bad play.
The problem with a back that plays neither on third down NOR gets goaline carries is you eliminate upside while also providing big chances for 3 point weeks. Third down backs will be in there even if their team is down. Goaline backs always have the chance to get 6 easy points.
So it is less the percentage of carries and more how the roles - main ball carrier, goaline back, third down/receiving back - are split