The reason it never go half way because you drilled the hole. Someone did this on a MB and the temperature gauge would always read very cool... until I change it again with no hole and it is back to normal.
That little hole makes the big difference in temp control because it allows hot coolant to escape to radiator when it is not supposed to. And because of extended idling, the temperature stabilized itself. As usual, when you start to drive again, too cold.
You are wasting too much gas due to cool engine.