Mixing Mogas with 100ll

Since there are 10 octane points between the two fuels, every 10% of the mixture that's 100 octane will raise the final octane by one point.  Thus, to get 91 octane you need 100LL to be 10% of the mixture.  Proof below.

Formula: (Fuel A Octane x Fuel A %) + (Fuel B Octane x Fuel B %) = Desired Octane

If n represents the percentage of fuel that is 100 octane, then the percentage of 90 octane is (1 - n).

90 x (1 - n) + (100 x n) = 91  [re-write formula]

90 - 90n + 100n = 91  [expand]

90 + 10n = 91  [simplify]

10n = 1  [subtract 90 from both sides]

n = 0.1  [divide both sides by 10]

∴ 100 octane = 10% of mixture