I took a brief look at the video. It appears that he is just encouraging the use of the theoretical Round Earth model on paper to determine a range

That's right. I assume, as a marine instructor, he knows what he is talking about.

In fact, applying that table to Rowbotham's example, I get 6.7 statute miles for the visible range of the boat at 24 feet height, plus 11.3 miles for the 68 feet of the lighthouse, giving a total maximum distance of 18 miles.

So Rowbotham's figure of 30 statute miles is way off. I wonder if he was just making this up? Do have evidence he was telling the truth?

Rowbotham and Co. took their information from credible sources of the day. Dubey writes about the matter:

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A copy of the book “The Lighthouses of the World” and a calculator are enough to prove that the Earth is not a globe, but an extended flat plane. The distance from which various lighthouse lights around the world are visible at sea far exceeds what could be found on a globe Earth 25,000 miles in circumference. For example, the Dunkerque Light in southern France at an altitude of 194 feet is visible from 28 miles away. Spherical trigonometry dictates that if the Earth was a globe with the given curvature of 8 inches per mile squared, this light should be hidden 190 feet below the horizon!

The Port Nicholson Light in New Zealand is 420 feet above sea-level and visible from 35 miles away which means it should be 220 feet below the horizon. The Egerö Light in Norway is 154 feet above high-water and visible from 28 statute miles where it should be 230 feet below the horizon. The Light at Madras, on the Esplanade, is 132 feet high and visible from 28 miles away, where it should be 250 feet below the line of sight. The Cordonan Light on the west coast of France is 207 feet high and visible from 31 miles away, putting it 280 feet below the line of sight. The light at Cape Bonavista, Newfoundland is 150 feet above sea-level and visible at 35 miles, where it should be 491 feet below the horizon. And the lighthouse steeple of St. Botolph’s Parish Church in Boston is 290 feet tall and visible from over 40 miles away, where it should be hidden a full 800 feet below the horizon!

“The distance across St. George's Channel, between Holyhead and Kingstown Harbour, near Dublin, is at least 60 statute miles. It is not an uncommon thing for passengers to notice, when in, and for a considerable distance beyond the centre of the Channel, the Light on Holyhead Pier, and the Poolbeg Light in Dublin Bay. The Lighthouse on Holyhead Pier shows a red light at an elevation of 44 feet above high water; and the Poolbeg Lighthouse exhibits two bright lights at an altitude of 68 feet; so that a vessel in the middle of the Channel would be 30 miles from each light; and allowing the observer to be on deck, and 24 feet above the water, the horizon on a globe would be 6 miles away. Deducting 6 miles from 30, the distance from the horizon to Holyhead, on the one hand, and to Dublin Bay on the other, would be 24 miles. The square of 24, multiplied by 8 inches, shows a declination of 384 feet. The altitude of the lights in Poolbeg Lighthouse is 68 feet; and of the red light on Holyhead Pier, 44 feet. Hence, if the earth were a globe, the former would always be 316 feet and the latter 340 feet below the horizon!”'—Zetetic Astronomy, Earth Not a Globe!” (59)

“The lights which are exhibited in lighthouses are seen by navigators at distances at which, according to the scale of the supposed ‘curvature’ given by astronomers, they ought to be many hundreds of feet, in some cases, down below the line of sight! For instance: the light at Cape Hatteras is seen at such a distance (40 miles) that, according to theory, it ought to be nine-hundred feet higher above the level of the sea than it absolutely is, in order to be visible! This is a conclusive proof that there is no ‘curvature,’ on the surface of the sea - ‘the level of the sea,’- ridiculous though it is to be under the necessity of proving it at all: but it is, nevertheless, a conclusive proof that the Earth is not a globe.”—William Carpenter, “100 Proofs the Earth is Not a Globe” (5)

The Isle of Wight lighthouse in England is 180 feet high and can be seen up to 42 miles away, a distance at which modern astronomers say the light should fall 996 feet below line of sight. The Cape L’Agulhas lighthouse in South Africa is 33 feet high, 238 feet above sea level, and can be seen for over 50 miles. If the world was a globe, this light would fall 1,400 feet below an observer’s line of sight! The Statue of Liberty in New York stands 326 feet above sea level and on a clear day can be seen as far as 60 miles away. If the Earth was a globe, that would put Lady Liberty at an impossible 2,074 feet below the horizon! The lighthouse at Port Said, Egypt, at an elevation of only 60 feet has been seen an astonishing 58 miles away, where, according to modern astronomy it should be 2,182 feet below the line of sight!

"The distance at which lights can be seen at sea entirely disposes of the idea that we are living on a huge ball.” —Thomas Winship, “Zetetic Cosmogeny” (58)

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He is teaching people who will 'test it in real life' in circumstances that will hopefully save their life!

If so, then as marine layer visibility can vary, better to assume short than long.