In 2015 a 1 loss ND was No. 4 until they crapped the bed against BC and Stanford. Play better in those games and I think they are in.
In 2017 a 1 loss ND was No. 3 going into Miami...win out and that team is in for sure.
In 2018, it would have come down to tOSU or ND. Depending on when and how ND lost I think they could have still been in with one loss since tOSU had such a bad loss against Purdue.
I think it's safe to say a 1-loss ND team will finish anywhere between 3 and 5. Obviously the difference between 4 and 5 is huge.