The old man's calculation was common sense. The world of cultivators was similar to the world of beasts. Even if you could kill another, it was meaningless if you got badly hurt.
A serious injury made you easy prey for others, and if your foundation was damaged, severing your future path, crying out to the heavens would be of no use. Therefore, unless one was certain of great gains, or if there was some irreconcilable grudge, disputes between cultivators rarely reached a fight to the death.
The old man believed that since he was a Foundation Establishment cultivator and his opponent was as well, there would be some hesitation in both their hearts. That was why he planned to use the formation to seize the initiative, pressure his opponent, and force a concession.
He was even prepared to give up all the spirit stones in his possession if his opponent proved stubborn. Of course, he had no intention of conceding that much unless his opponent came at him with a 'you die, I die' attitude. It was merely a thought in preparation for the worst-case scenario.
“Fortune smiled upon me today. The situation is against you, so it would be no shame for you to yield a little.”