I have the same issue. Only 89 available without ethanol in Charleston, SC. I do mix but not by any exact means. There’s likely always 50% 89 and 50% 100ll but I’m certain it’s occasionally swung 80/20 in both directions. Never had any issue with my 912is. When I’m running this mix it is usually the winter and not all that hot. Maybe max 85 F. I think there’s less chance of knocking in cooler temps if the octane is too low, correct me if I’m wrong. At least that’s how I justify.

I went down the finding the recipe route. The issue I found is that there’s no easy conversion between the two ways of rating octane or anti knock index. Also I believe the way lead changes the octane is non-linear.