Your anchor alarm is only as honest as the numbers you give it. Those numbers start with scope.
Scope, quickly
Scope is rode length divided by depth-plus-freeboard. At 3:1 an anchor holds poorly; 5:1 is a reasonable settled-weather minimum on chain; 7:1 or more is where you want to be when it pipes up. More scope flattens the pull on the anchor, and flat pulls hold.
From rode to watch radius
At anchor the boat swings on an arc whose radius is roughly the horizontal component of your rode. In 4 m of water with 30 m of rode out, you'll lie up to about 29 m from the pin in any direction the wind chooses. That's why Sailing Log asks for your rode length and adds a margin (20% by default) instead of asking you to guess a radius: 30 m of rode becomes a 36 m alarm circle, drift past it and the siren goes.
Honest edge cases
- Wind shifts: the pin is where the phone was when you armed, not where the hook lies. A 180° shift can swing you toward the circle's far side; the margin absorbs most of this, and you can widen it in gusty anchorages.
- Tide: deeper water at high tide shortens your effective scope. Set rode numbers for high water, not the moment you dropped.
Set real numbers, arm it, sleep. That's the entire workflow.