What it works out
How many gallons of E85 to pump first, then how much pump gas to top off with, so a full tank lands on the blend you want. It is a volume mix: fuel in now × its % + E85 gal × its % + pump gas gal × its % = tank size × target %, solved for the E85.
Why the E85 number varies
ASTM D5798, the spec for pump E85 since 2011, allows anywhere from 51% to 83% ethanol, so the same fill from a 51% pump and an 83% pump lands on two different blends. The tool defaults to 80%; if you can test the pump's fuel, type what you measure.
Filling in the inputs
Fuel in the tank now is your best read of the gauge, in gallons. If the car has been on E10, leave its ethanol at 10. Target blend is the number after the E, so 30 for E30. Pump gas is usually 10, or 0 for ethanol-free.
Reading the result
With the defaults, a 14 gal tank with 3 gal of E10 left, E30 target and 80% E85: add 4.0 gal of E85, then 7.0 gal of pump gas, for a full tank at E30. If the target is out of reach the tool says so and shows the best it can do: from that same 3 gal, E85 alone gives about E65. The math assumes you fill all the way. Mix only what your fuel system and tune are set up for.