If Vaan consumed more of the Divine-rank Stalactite Milk, the potency would be mostly absorbed by his meridians and five viscera before he could direct it into his bones.
Thus, oral consumption wasn't effective if he wanted to temper the bones solely.
Fortunately, Vaan had comprehended the Spatial Law and overcome his limitations with the Heaven-Swallowing Space. He could open a small portal and directly infuse the Divine-rank Stalactite Milk into his bone marrows.
Nevertheless, the bone marrow was different from the meridians and five viscera. Without any records of the Stalactite Milk being used on the bone marrow, it was difficult to determine the exact effect it would have.
As such, Vaan infused the diluted version, the Transcendent-rank Stalactite Milk, into his tailbone, which was considered the most useless bone in the human body. As such, even if something went wrong, he wouldn't suffer any loss.
