For the longitude about 4 seconds translate into a minute of arc due to the earthīs rotation. So 5 minutes of arc would translate into 20 seconds. Errors in the measuring of the elevation go on top of this.
Depending on the vessel 5 minutes of arc may the all accuracy you get with an accurate timepiece.
The calculation for cel-nav takes the known deviation of the timepiece into account. Thatīs why certified timepieces do not only have a maximum acceptable deviation per day but also for different orientations.
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If it isnīt broken, it doesnīt have enough features yet.