This is correct. It can use up to 96 octane. - so with 110, perfect ratio to get 96 would be 1/3 110 and 2/3 89. Otherwise, 1/4 110 and 3/4 91 will get you just a tick under 96, give you pretty much all the gains of the higher octane while keeping a little more $ in your pocket.I think the ECU will only be able to use up to about 95-96 octane. More octane helps against predetonantion some and allow for some advanced timing, but not infinitely.
I guess the best way to really determine is to do some dyno runs with different mixes to get different octanes and see where the gains stop. I would think 3-4 gallons of 110 would be enough on a full fill-up.