You first determine what aperture you intend to use. Than you set that aperture on both your camera lens and the flash.
When the flash is set to an ISO of 1600 and an aperture of f/16, the "23 feet" you see displayed is an indication of the expected maximum flash-to-subject distance you can expect to work with. Changing either the ISO or the aperture will affect that maximum, but the TTL circuitry in the flash and camera will automatically adjust the light output for shorter distances.
You can adjust the "f/16" by using the controls on the back of the flash. When you do, you will see that the "23 feet" will change.
Maximum range calculations are somewhat dependent on the scene - low, light coloured ceilings and reflecting floors and walls really assist a flash. A flash confirmation function is available on the flash.
Here is a link to the manual: http://www.summilux.net/documents/LeicaSF20.pdf
The pages are quite small, but they work with the zoom function.