Traditional strength of schedule calculations add up the record of a team’s 16 opponents to get a cumulative win percentage for all 16 opponents. That exercise can also be done mid-season to get a feel for the remaining strength of schedule (SOS) each team faces.
For example, the Colts' nine remaining opponents (counting division opponents twice) have a 31-21 W/L record so far this season, so the Colts have an SOS of .596, the highest of any team in the NFL.
Another way to look at remaining SOS is to use the Vegas Super Bowl odds as a proxy for team strength.