It is well known that three or four times the maximum expected depth of water is a good starting-point for deciding scope when anchoring with chain cable. This rule is not cast in stone, however, and you might safely may opt for less cable if you are tight for space, so long as your anchor seems well set, conditions are not extreme and you will be aboard at high water.

This can be particularly useful when swinging room is tight and the tide is due to fall during the night, because you know then that your scope will increase for some hours before it begins to decrease once more. If your anchor is holding at bed-time, it should hang on until morning light.