No. That would be for a full swap, even temporarily.

It's hard determining a mix, because a lot of stations are using winter "E85", which can be lower on ethanol content. They make ethanol test kits to determine the ethanol at the pumps.

For a mix, you're probably not going to see much of a result, depending on what your regular gas is. Around here most 91 is 10% ethanol for example, so if I did 15 gallons of E10 and 5 gallons E70, it would come out to 20% ethanol.

This site has a pretty sweet conversion calculator. https://www.intercepteft.com/calc.html