What's in the tank, what you want. It tells you how many gallons of E85 and pump gas to add.
Illinois E85 runs about E70 in winter and can legally be E51. If you have a sensor, pump a gallon and read it.
Estimates. Pump E85 varies by season, tank sizes are spec-sheet numbers. A flex fuel sensor is the only thing that actually knows.
Most off-the-shelf Stage 1 and Stage 2 maps for the N54, N55, B58, S55 and S58 are written for E30 to E50. Below that you leave power on the table, above it the stock fuel system starts to run out on the higher-power cars. With a flex fuel sensor the tune reads the actual blend and adjusts, so you can run whatever the pump gives you.
Pump E85 is a seasonal blend. In Illinois it is close to E85 in summer and drops to around E70 in winter so cars start in the cold. Legally it only has to be E51. If you blend by the math, use the real number; if you have a sensor, fill a gallon and read it.
Not on a pump-gas map, and not on the stock fuel system once power climbs. E85 needs about a third more fuel than gasoline. On the B58TU and N54 the answer past about E60 is port injection; on the S55, S58 and the V8s it is a bigger high-pressure pump. The Build Lab walks through what each engine needs.
The math is exact; the inputs are not. Pump E85 content varies by season and by station, tank sizes are nominal, and the car's reading moves a little with temperature. Treat the result as a recipe that lands within a few points of the target, and let the sensor do the rest.