Paul, good question. The duty cycle the DRV8308 applies during startup is not well-controlled. The SPEED register does affect it, but really just needs to be a large enough value to spin the motor up to the target speed. Lower values of SPEED should prevent overshoot, but if it's too low, target speed won't be reached, so it's simplest to just set it to 4000.
Best regards,
RE
Best regards,
RE