What is the most energy-saving setting for the outlet temperature of gas water heater?

The lower the outlet temperature of gas water heater, the more energy-saving it is, but the requirements of bathing should also be considered.

The specific operation is as follows:

Set it to the desired water temperature (such as 40 degrees), and put hot water directly when using it.

Set a higher temperature (such as 60 degrees) and use cold water.