How many grains of millet could Joseon Dynasty sacks hold?

The writing on the above container translates as follows:

穀 (곡) 用 (용), 五 (오) 작 (勺)
grain (穀) use (用), five (五) jak (勺)

"Grain (穀) use (用)" means the above container was used to measure grain instead of liquids, which were measured in separate but similar containers.  "Jak (勺)" refers to the unit of measure, so the above container could hold five (五) jaks of grain. But how much was a jak (勺)?

According to THIS ARTICLE, a jak (勺) was a container used during the Joseon Dynasty that could hold 1,200 grains of millet (기장). That means the 5-jak container above should hold about 6,000 grains of millet (5 x 1,200). So how many grains of millet could the large straw grain sacks (섬) of the Joseon Dynasty hold? Let's do the math.

The following were the units of volume measure used during the Joseon Dynasty:

• 勺 (작) = 1,200 grains of millet
• 合 (홉) = 10 작, so 12,000 grains of millet
• 升 (승) or 되 = 10 홉, so 120,000 grains of millet
• 斗 (두) or 말 = 10 되, so 1,200,000 grains of millet
• 石 (석) or 섬 or large sack = 20 말, so 24,000,000 grains of millet

Therefore, based on the linked article, a large, straw grain sack (1 섬), similar to the one shown below, should have been able to hold about 24 million grains of millet. Apparently, there were also small grain sacks that could hold 15 말, which would be about 18 million grains of millet. However, I have read elsewhere that one 섬 equaled 10 말, which would mean one 섬, or sack, could have held about 12 million grains of millet.

One final question: Why did they choose 1,200 grains of millet to equal one jak (勺)? I would guess that one jak of millet equaled about one cooked serving of it.