Here's a guide I wrote to get E47 but you can use the App for any combination:
Extra 1: How to mix ethanol guide
So at first I was really scared/confused about how ethanol mixing really worked but now I know that its easy as pie. Here's how you mix your first tank of E47...
Step 1: Let the car get below half a tank
Step 2: Add in 7.1 (Different on Mk7's) gallons of E85
Step 3: Top off the tank with 93/91 octane gas
Now you have E47.
Your preceding tanks take a bit more work but barely. Download an app called "E85 Mix Calculator". Our fuel tank is 14.5 gallons large. So basically you just estimate how much you have left in your tank and let the calculator do the rest, it's easy! Below is a picture of it in action.
Really it's easy. Tunes are conservative anyway to account for differences in pump gas, but Id rather have a conservative tune for 105 octane with massive cooling and knock resistance than a conservative tune for 93 octane.