If ND plays 5 ACC games they would still play Navy and USC (no brainers), and most likely Purdue and MSU.
Michigan and Stanford rounds out the schedule to 11 games.
Can also play schools like Purdue, MSU, UM, and Stanford 4 times each, every five yrs., and fill in the open yr. with schools in other geographic locations to keep the alumni happy.
For all intent and purpose they play 3 ACC schools this year: BC, Miami and Pitt (who will join the ACC next season).
If they do join a conf. would think Big 10 makes more sense (certainly from a geographic point of view) but maybe they think they would fare better in the ACC where the football is good, but certainly not great.