Youshen had basically determined that, at the present stage, continuing to improve his physical fitness would not yield any further increase in the optimal age limit for his spirit rings.
Not to mention the physical strength of the Youxing character card at the Spirit Emperor level, perhaps even a body with merely the strength of a Spirit King had already reached the upper limit of this formula.
According to Youshen's speculation, the deductive formula for the limit of the second spirit ring was: Spiritual Power starting from the Spirit Abyss Realm + Spirit King physical strength + ? Spirit Power Level ≈ Lower limit of the spirit ring age for the seventh spirit ring of a Spirit Emperor from the old era.
Chapter 87