Always measure anchor depth from deck rather than waterline, your anchor is attached at deck level.
In tidal water, calculate depth at high tide.
Some sounders are set to display depth under keel, and you'll have to add the draft.
Where is my anchor?
Open your favorite anchor alarm. You'll see your boat location on the map. You can now place the anchor on the app at its bearing and distance form your position.
Bearing : Your anchor is in the direction of the wind (or tide if any), which is also your average compass heading.
Distance: You are now back in the aft cockpit, the distance to your anchor is about boat length + chain length (In a center cockpit add your distance to the windlass to chain length).
There is no need yet to place the anchor very accurately on the map.You'll have plenty of time to adjust the anchor position later. After you launched the app, your GPS accuracy is still quite low anyway.
Which alarm radius?
Always add some margin to your anchor distance because of GPS inaccuracy. A 10m or 30ft margin is the minimum to avoid false alarms and 15m / 45ft is recommended.
If your app filters inaccurate positions, make sure the required accuracy in the filter is consistent with your margin. The margin and filter values don't need to be equal but should be close.
The good margin
Ideally, the margin is above the expected (in)accuracy. You'll get few false alarms.
However, the depth must be sufficient in the false negative zone that surrounds your alarm zone. If the false negative zone is not deep enough, you'll have to reduce the margin. It's your call to define the acceptable trade off between false alarms and timely alerts.
Maintain the watch
You should check periodically your position relative to the circle in the app and keep an eye outside as you were taught while learning sailing.
As a boat Master or Skipper, you are responsible to keep the crew and the boat safe. No electronic aid nor app will relieve you from your responsibility nor liability. Electronics aids and apps offer an aid but they might fail, be inaccurate, be unavailable... yet you're still in charge.