My local Safeway started carrying B20. I'm thinking of starting with a diluted amount. I assume mixing would occur naturally in my tank if I added additional fuel.

So, if I have 5 gallons of B20, and add 15 more gallons of regular diesel, do I now have B5?

I'm assuming that in the original B20, 1 gallon is essentially Bio. So, if that is correct, adding more would make B5 or 1 of 20 gallons.