Rex has learned about the Sanity stat on his first-ever experience of a full moon after he was no longer a human. He knew the Sanity stat would be calculated during each full moon and it would determine whether he would lose control or not.
It was measured through his current state of mind.
Positive influences would increase his Sanity stat and the same for the opposite.
Keeping it above 50% is preferable as that would stop him from losing control.
But the moment he became a High Werewolf and then eventually a Royal—the influence of a full moon had less effect, on his mind, and the Sanity stat became less strict. Even if it fell to 30% during a full moon, he wouldn't lose control that easily.
Not to mention there was Evelyn now who could help him boost his Sanity stat as his Luna.
As time passed, it became hard to drain his Sanity stat to below 30%.