@ShardzWeaponWith how the current system work, it's going to be very tricky for casual players to find the right difficulty setting after season 1 (because of how much the AI drivers improved from their hidden facility boosts).
From my personal experience of past games, I usually had to drop at least 5 points (from 100-110) on the difficulty setting for many tracks from season 2 onward. Also, unless your teammate is also hitting 100 ratings from the facility boosts, don't use him as your standard for setting difficulties, or else you're gonna struggle real bad against other teams since most of their drivers are near maxed out at that point.