For best results, I always use the factory recommended range, that would be in the 190-195º range for a stock engine.
However, it may not be the thermostat that is faulty. It is quite possible the cooling system is sucking air in somewhere when it cools. That air forms a pocket against the thermostat and won't open the thermostat like water would. This is why many will recommend drilling a small hole in the thermostat so the air bleeds through. But my experience has been once you're getting air in the system, it's a good idea to find out why. it could be as serious as a head gasket or as simple as a as radiator cap or leak hole or weak hose clamp. So I'd recommend a two step approach - put a new thermostat with a bleed hole in and then pressure test the cooling system for leaks.
However, it may not be the thermostat that is faulty. It is quite possible the cooling system is sucking air in somewhere when it cools. That air forms a pocket against the thermostat and won't open the thermostat like water would. This is why many will recommend drilling a small hole in the thermostat so the air bleeds through. But my experience has been once you're getting air in the system, it's a good idea to find out why. it could be as serious as a head gasket or as simple as a as radiator cap or leak hole or weak hose clamp. So I'd recommend a two step approach - put a new thermostat with a bleed hole in and then pressure test the cooling system for leaks.