
The
Rule of
Twelfths.
The Rule of tenths. The Calculation. Some other rules of thumb. Caveat. 
For working out accurate tidal heights you should use a graph.
However, the rule of twelfths allows you to make a fair estimate quickly in your head.
The rule works on the assumption that the tidal flow increases smoothly to a maximum at half tide then decreases smoothly to zero flow at slack water.
And the flow rate correlates to the increase or decrease in depth over the approximate six hours of the tide sequence.10% for the 1st hour of range
15% for the 2nd hour of range
25% for the 3rd hour of range
25% for the 4th hour of range
15% for the 5th hour of range
and 10% for the 6th hour of range