Koku in Dungeons & Dragons: for Daimyō

My previous post dug out a conversion rate of 25 gold pieces (GP) for one koku. That post focused upon the minimum income of a single samurai. As we have a historical record of the kokudaka (tax assessment of the entire country), I can expand my conversion to the daimyō—the landholding baronial class of pre-modern Japan. Under the kokudaka:

• The minimum annual revenue needed to be considered a daimyō was 10,000 koku. So 250,000 GP in agricultural production, annually.
• Many daimyō had revenue in excess of 100,000 koku. 2.5 million GP.
• The future Shōgun, Tokugawa Ieyasu, was daimyō of the eight provinces of the Kanto plain which produced an annual revenue of 2.5 million koku. 62.5 million GP.
• Out of a (1598) national total of just under 19 million koku. That is 475 million GP! Per year!
• Most of that was eaten, of course; Japan’s population at the time was somewhere around 19 million.