The best ratio for gas mixture is 14.7 parts air to 1 part fuel.
The best ratio for e85 mixture is 9.8 parts air to 1 part fuel.
The O2 sensor does not know how much air or fuel you started with. It only knows oxygen levels left in the spent charge. The O2 sensor reports in terms of lambda. If the lambda value is 1.0, the fuel is burned optimally, and started with the right mix. Whether you start with gas at 14.7:1 or e85 at 9.8:1, the lambda will be the same at the O2 sensor.
When you see a value reported from a wideband, the value is typically converted to a gas equivalent AFR. Some widebands (like the PLX) will let you change the display and show lambda, or show ethanol-equivalent AFR's. So, let's say your wideband is set to show gas equivalent AFR, but you are running e85. Cruising around, the display will show 14.7:1. This means the lambda is 1.0, and in reality your e85 mix is 9.8:1. When you go WOT, the wideband display shows 11.76:1. This means your lambda is .80, and if you are running e85, the actual mix was 7.84:1. (Narrowbands work in the same way, but we only see raw voltages).
When you switch to e85, you have to tell the computer to increase the fuel delivery to make the initial mix 9.8:1. If you change the stoich setting in HPT, you must also change other fueling tables to make sure the calculations stay correct. (See this post for more info:
http://www.hptuners.com/forum/showthread.php?t=31925). A simpler method is to tell the injectors to just deliver more fuel. This keeps all the other calculations correct, and only requires changing one table. The rule of thumb is usually to start out by telling the injectors to deliver 30% more fuel, and then tweak from there.
To run leaner at cruise, you must work around the fact that the computer is always trying to compensate off the lambda. If you just change injector values, the computer will still try to trim the fuel delivery to meet the stoich value in the closed-loop feedback. That is the big benefit of going all open-loop. You decide exactly how much fuel to deliver based on airflow, not feedback. If you want to lean out and stay in closed loop, follow the stoich changes described in that link above.