The slack window is not an instant
A tide table gives you a time. The water gives you a period, and the two do not match.
Why the table looks exact
High water is printed to the minute because the table has to print something. The prediction behind it is a harmonic model with a stated tolerance, and near the top of the tide the height curve is flat enough that a ten minute error changes the water by a few centimetres. That flatness is what makes the printed time look trustworthy and hides the fact that the usable period has edges.
The window scales with range
The length of time the current stays weak is roughly proportional to the range. A 5.2 m spring gives a wider slack than a 2.1 m neap on the same day of the cycle, even though the printed high water times are only minutes apart. This is why a plan built on one neap day quietly falls apart on the next spring.
What to carry on the boat
Carry the earliest and latest time as a pair of numbers in marker on the deck card. A pair forces a decision at the earlier end, which is where decisions belong. A single centre time invites a push, and the push is what turns a marginal gate into an incident.
A worked example
Take a port with high water at 11:20 and a 4.8 m range. On a calm day the centre of the window sits about ten minutes before the printed time, with usable water twelve minutes either side. Wind opposing the current can remove a third of that at the ends. Write 10:58 to 11:22, then shorten it yourself when the forecast turns.