For a race car, buy E85 in a drum!
Pump fuel will vary the ethanol content depending on market, weather ect also the other unleaded part will vary fron 91, 95, 98 ect
From caltex: When Caltex retails the fuel, it will include between 70 and 85 per cent ethanol blended with regular petrol, depending on the time of year.
All good drum E85 will be 85%ethanol and 15% 98 fuel mix
Also a drum is sealed and will store for ages, servos it wont last long at all, its vented to the atmosphere, ethanol mixed fuel can absorb water from the atmosphere about 50 times more then normal non alcohol fuels, less then 1 tablespoon per gallon of water will spoil the fuel and start separation, as water will not blend with petroleum
So i wouldn't leave it sit in the tank either, drain, fill with 98 ect, run the car for a little, then fill up with fresh E85 next track day.
Few places around, for example race fuels in Melb you can bring your own drum and fill up with good race 'drum' E85, obviously cheaper then buying drums from places like vpw, rocket ect, saves cost of the drum, couriers ect.