You can do this already, but it might be a bit confusing. SUppose this time youre filling 70%eth/30%gas:

Fill some ethanol, enter a new record:
MIles: your car odomoeter miles
Trip: ignore (fuelpad calculates)
Fill: how much ethanol you filled
Price: How much $ you paid for eth
Not Full Tank: YES (TICKED)
Notes: 70%eth/30%gas

Fill some gasoline, enter a new record:
MIles: your car odometer miles again, same as before
Trip: ignore (fuelpad calculates)
Fill: how much gas you filled
Price: How much $ you paid for gas
Not Full Tank: NO
Notes: 70%eth/30%gas

You now have:
total $ for this time
total litres for this time

This is the first record, there is no MPG

Drive

Fill again, lets say this time 80/20


Fill some ethanol, enter a new record:
MIles: your car odomoeter miles
Trip: ignore (fuelpad calculates)
Fill: how much ethanol you filled
Price: How much $ you paid for eth
Not Full Tank: YES (TICKED)
Notes: 80%eth/20%gas

Fill some gasoline, enter a new record:
MIles: your car odometer miles again, same as before
Trip: ignore (fuelpad calculates)
Fill: how much gas you filled
Price: How much $ you paid for gas
Not Full Tank: NO
Notes: 80%eth/20%gas

You again have your totals for this time

but the MPG relates to the mix you had LAST TIME because that is the fuel you used to travel up to this mileage

If you want to easily work out your best $/km cost you'll need to wait for a bug I just filed to be fixed but remember the price/distance column related to the fuel you bought LAST TIME