If you plan to observe with a telescope, you can select 'Other' and enter the limiting magnitude yourself. This is the minimum height above the Earth's horizon. If your horizon is obstructed by nearby homes, trees, or mountains, select 'Plus 20'. The idea is to predict rise and set times that are closest to the time you will actually have a clear view of the satellite. This determines how bright the sky is at the beginning of your prediction 'window'.
Civil twilight occurs just after the sun has set and the sky is still fairly light.
Nautical twilight is still lit up a bit. As a general rule, observations get more difficult as you move down these three categories. Accurate Time There are many sources to choose from when synchronizing your watch. I use a similar service, broadcast on shortwave radio. Your television news might even display the time with seconds.