Determine it from what? The 7.5us minimum pulse rate? If that is the length of two segments of the Stepper.h quadrature cycle, then Stepper.h would need to step twice as fast, or 3.75us/step, or 1/3.75e-6=266666.7steps/sec. I'm not sure how fast stepper.h can go, but 267kSteps/sec or 266666.7/1600*60=10000 RPM seems pretty high. I'd bet the overhead of Stepper.h limits it quite a bit.