The back face is for navigation with the wind triangle methods: Direction and speed vectors of wind and aircraft give a vivid picture of all your manoeuvres and the corresponding position of your aircraft. The wind and ground speeds as well as coupling a number of legs are easily calculated.
Speed ranges on the two chart boards: 20 - 200 and 150 - 850 km/h, mph or knots.

In aviation, absolutely reliable and safe navigation aids are vital. Thus, mechanical calculators and navigation devices have claimed their place next to electronics, because they offer constant availability, without any risk of failure due to depleted energy sources or technical defects.