Source: Simple math. Pick a site that publishes some energy numbers.
http://bioenergy.ornl.gov/papers/misc/energy_conv.html
Ethanol = 75,700 Btu/gallon
Gasoline = US gallon = 115,000 BTU
Gallon of E10: 0.1*75,700 + 0.9*115,000 = 111,070 BTU.
Gallon of E10 will have 111,070/115,000 = 96.6% the energy of a gallon of gasoline which should translate to a 3.4% loss of efficiency. Put this formula into a spreadsheet and you can calculate the percent loss for any concentration of Ethanol.
Everyone gets a different calculated loss because the published BTU for gasoline varies.
The loss should be 3% to 4%. Unfortunately in practice the loss claims seem to average around 10%. Some claim a small improvement and some claim as high as 30% loss. Any loss above 10% the ethanol is not only providing no value, it's forcing you to burn more gasoline than you would have if the ethanol were discarded. When the non renewable fossil fuel, water, and increased small engine service costs are included the break point is much less than 10%. There isn't much room between 4% and 10%.
How this helps pollution or the economy is beyond me. Even if no one ever saw losses above the theoretical ~4% I would not consider Ethanol for fuel a good deal.