How to use correctly a microstepping driver with a lower voltage stepper motor
That doesn't matter at all.
The voltage stepper motors are rated for is the voltage that, if applied to the motor coils, would create its rated current.
Stepper motor drivers monitor the current, and always use the full voltage available to them (9 - 40V in your case) to drive the stepper motor - using PWM.
The stepper motors coils inductance slows down the change in current, allowing the driver to reach its set current.
Higher drive voltages simply mean that it's easier to overcome that inductance, allowing for faster stepper motor movements.
You'll want to set your driver at 1.0 or 1.5 A, and supply it with as much voltage (up to the rated 40 V) as you can.