The rule of thumb is about 30% more fuel. I beleive this is closer to 20%. My z is around 650HP with my 740's maxed out on E85. I can say that it is a very forgiving fuel. I have ran way lean without any detonation! I don't use a Flex fuel sensor, I have dual widebands and Nistune. If I have to fill up at a different station than normal I just watch the WB's for any changes. If it runs leaner/richer all I have to do is enrichen/lean out my fuel map with the push of a button on the laptop. It only takes a few seconds to adjust. ;-)