Well it's not science so there technically isn't a correct answer. But a logical perspective on the cinematics show that the OPs original mathematics (although correct based on the image of the ship in the rings) does NOT fit logically into what is depicted in the cutscenes. The Dreadnaught in the cutscenes is much closer to the size of a small city based on the size of the Tomb Ships. Probably almost akin to New York City and the island of Manhattan.