Question for those who have flex fuel kit
 

I will have mine installed soon. What percentage of different types of mixed fuel (all petro, mix of petro and ethanol, all ethanol) would you have in the tank after installing the kit? Is a certain percentage of mixture of the fuel type that is best for the kit and my car? Or is it really up to me, there’s no harm whatsoever of the fuel mixture?

100% gasoline (of appropriate octane) to E85 or anywhere in between. I usually drive easy for a mile or two after a fill up for the sensor to read, but in reality it's almost instantaneous.

Let’s say initially u filled up the tank with all petro, at the half way mark u filled it up with half tank of ethanol, can you feel the performance difference?

Yes, as long as you have a tune for the ethanol.

For my car anything above 50% Ethanol content feels the same power wise... going by my anecdotal ass dyno opinion.

I’m getting the Delicious Tuning FF kit and their tune.

That's exactly what I have (albeit w/Harrop supercharger too), and the difference is very noticeable.