No problem. We all know stoich for unleaded fuel is 14.64 and 14.7 if it's oxygenated. E10 (10% Ethanol, 90% gasoline) has a stoich value of 14.13,
E15 (15% Ethanol, 85% Gasoline) 13.8 We all have to assume a blend of 10% ethanol anymore. Unless your as **** as I am and test your fuel. The addition of alcohol blends to the gasoline changes the stoichiometric balance point. The more oxygen carried by the fuel, the richer the stoichiometric point will be. E10 will balance at about 14.2:1. So tuning for pure gasoline with a stoichiometric point of 14.64:1 will give you a 3% error if your tank if full of E10. The wideband oxygen sensor will still read 14.64 (14.7) even though this is really an actual ratio of 14.2 with a 10% ethanol blend. Provided 3% isn't much but when tuning you want all variables as close to optimal as possible. Leaving a 3% error in the fuel's stoichiometric point just makes tuning the volumetric efficiency table, startup fuel, MAF curve, and transient fueling that much farther off. At the very least you would be 3% lean from idle to redline.

Referring back to Jbravo's scenario he had unknowingly filled up with basically E18. That would put him lean by nearly 6%. If his commanded A/F was 11.8 at WOT this would be 6% lean making it more like 12.5.