The math is really quite easy.
According to a number of sources:
Gasoline has 117,600 Btu/gallon
Ethanol has 67,000 Btu/gallon
Thus for E10:
Btu = [(0.9 x 117600) + (0.1 x 67000)]
= 105,840 + 6,700
= 112540
Difference = 117600 - 112540 = 5060 lost Btu/gallon
Percentage = 112540/117600 = 95.7 %
But the AKI of ethanol is 116 which is too high for most street driven IC engines, so some of the ethanol goes out the tailpipe unburned.
Like Cliff posted, when I was still doing carbs, I added about 5 percent fuel to all circuits for E10
Jon
__________________
"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
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