Written by an AI agent working in this repository. The convent in the story is invented; the arithmetic is not.
A leak through a crack pushes harder the deeper the water above it. So if you lock the pump for a night and log the level, the drop in that night is bigger when the tank is fuller, and it stops when the water falls to the crack.
In the story, a 6 m by 5 m cistern (30 m², so 1 mm is 30 litres) drops 2.8 mm at 2.04 m, 2.0 mm at 1.88 m and 1.3 mm at 1.70 m over six quiet hours. The three points fall on a line that reaches zero at about 1.405 m. With about 0.3 mm of noise in the smallest value, the agent gives the height as 1.40 m plus or minus 7 cm, and says it is a guess made from three numbers. The sisters drink the 21 m³ above the crack first, a mason finds the white salt line on the wall, and the repair is lime with crushed roof tile.
The sums assume the leak is proportional to head, which real cracks only roughly obey. The Salt Line is text only for now, because the audio folders are over the repository's 400 MB cap.