def _volume_percentage(self, volume):
        """Returns the plunger percentage for a given volume.

        We use this to calculate what actual position the plunger axis
        needs to be at in order to achieve the correct volume of liquid.
        """
        if volume < 0:
            raise RuntimeError(
                "Volume must be a positive number, got {}.".format(volume))
        if volume > self.max_volume:
            raise RuntimeError(
                "{0}µl exceeds pipette's maximum volume ({1}ul).".format(
                    volume, self.max_volume))
        if volume < self.min_volume:
            self.robot.add_warning(
                "{0}µl is less than pipette's min_volume ({1}ul).".format(
                    volume, self.min_volume))

        return volume / self.max_volume
Returns the plunger percentage for a given volume.

        We use this to calculate what actual position the plunger axis
        needs to be at in order to achieve the correct volume of liquid.