For a 3S LiFe, what do you set your ECU low voltage cutoff at?

Its partly capacity/ C rating dependent. But The Li-Fe will generally collapse once each cell is below 3.2v so 9.6v.

With kero start turbines the biggest battery demand is start, when kero burner, starter motor and pump all kick in. If the battery supports this load generally it will run just the pump for the flight and then starter for cool down

Dave