Since the question is looking for the average current for the preceding 13 hours or so, I would be referring to the Tidal Current Tables in the back section that displays the current graph. If you inspect this table you’ll find a number of graphs that list points along your route and others eg; Chesapeake Bay, Hudson River, Long Island Sound, etc.
The colors/shading for “flood” and “ebb” are slightly different. On the side margin you’ll find “speedlines” marked with speed increments which are used by walking them with a parallel ruler onto the graph. If you walk the [B]9 knot[/B] line over your time at Carolina light, it will fall within or across the tidal current cycle and you should be able to determine an average current speed.
This graph is generally used when you want to know what “speed to use” to carry the current a long distance, say Chesapeake Bay Entrance to Baltimore. Or if you want to time your arrival for slack at a specific point along the route where you wish to avoid any adverse effect from too much current, like The Race at the eastern end of Long Island Sound…
It’s not terribly complicated, and it comes in handy. Read the instructions on the graph’s page and give it a try.
I hope this has been helpful.