Yeah this is not that hard.
My approach was always to run the tank down to about 2 gallons left and then always put in 13 gallons of the correct % mix.
You don't need to worry about calculating what % of ethanol is in the tank, just make sure whatever quantity you're adding is slightly less than available capacity and it's the right percentage.
If this is too complicated for you, you should rethink tackling these kinds of mods. Sorry not sorry...
