The rule of twelfths
Estimate the height between high and low with your fingers.
One-two-three, three-two-one
Between low and high, the water doesn't rise at a steady rate. It crawls in the first hour and floods in the middle. The rule of twelfths turns that into something you can do on your fingers.
hour 1 1/12 just starting hour 2 2/12 picking up hour 3 3/12 moving fast hour 4 3/12 still running hour 5 2/12 slowing hour 6 1/12 nearly full
Why it matters at anchor
If you anchor in two metres at low water and the range is four metres, you need to know the middle two hours add most of that rise. Scope set for low water can be too short by high.
It's an estimate, and that's the point
The rule assumes a perfect six-hour tide with no wind and no river. Real water never behaves. Use it to build a margin, not to bet the boat.