Just some supplementary information on this...

There are 52 events (46 trophy + 6 TT) and the progress rewards at 25-50-75-100% are 10-15-21-33GC. The last 11 cup races all need the final PR90 - one on tier 17, three each on tiers 19-21, and just one on tier 22 (which is seven laps at Le Mans paying 15,500 Fame).

Alternatively, if you're just looking to get to 75% your car/one of your cars needs to reach PR86.6 for the cup race on tier 14, which along with the four non-cup events tiers 16/17 is enough: