US 87 (R+M)/2 has minimum MON of 82, so minimum RON is 92 (91.5 absolute minimum RON with 82 MON).
If a facility blends with higher MON, then RON can be lower, but minimum MON is typically a greater extent to such an extent there are some facilties / times that (R+M)/2 giveaway occurs in order to meet minimum MON. So it's difficult to assign a single RON (or MON) value to a grade based on (R+M)/2. You can just benchmark off minimums.
Parrafins like normal butane, alkylate, and isomerate have a low (2 to 4) difference between RON & MON. Aromatics such as toluene and reformate / platformate typically have a 12 to 14 number difference between RON & MON. Olefins like hexene, isooctene, and nonene have a 8 to 10 difference between RON & MON. Nonidealities in interactions cause some components to blend differently than their "neat" properties as well.