It helps if your throttle has a discrete idle detent, but I find the easiest way to have reverse thrust using only the axis is to set the Neutral point in the response curve screen so that when the physical lever is at idle, the white dot on the response curve is right on, or a tiny bit below, the central horizontal line. You can also set a tiny 1% deadzone right in the middle if you find the throttle teeters on going into reverse accidentally.
No buttons necessary.