I'll be heading to the track on Wednesday with some 104 octane race fuel and a 100 octane tune but don't know how to properly mix the fuel. Do I have to run it to completely empty or get it to 10 miles to empty and then pour in the 100 octane?

The lower you get your tank the less it will cost to equal 100 octane.  Roughly speaking if you can get it down to 1/8 of a tank your looking at 2-3 gallons left.  So adding 3 or 4 gallons will net you are 99 to 100 actual octane in tank

2*93=186+3*104=312=498. Divide your octane by gallons

498/5=99.6 octane. 

For the sake of a gallon or two do you really want to risk mixing not enough and running with a lower octane rating?

Just sayin'