Evolution

for you guys who uses 100 octane with 91,how do you come up with the calculation for the mixed gas.ive tried xylene a couple of times and not much noticeable dirfference except for the smoother response.i wanna try this and see how it will work for the evo.

EuroEclipse

I personally have not mixed my gas, yet. As far as what's going on though, you're essentially creating something along the lines of 92-94 octane gas. (Rough estimate)
Reason being, when they produce the different octanes you can actually see the different layers of fuel in the mix. They seperate it, process it, and test it and market it for its octane rating.
So, with that in mind the most you're going to get is smoother response unless you tune for it. BUT you'd have to perfectly mix the gas the exact same way everytime if you want the same results everytime.