It should be 100% or at least above 70% of the full scale. Hot water on its own will give burnt hot water, and a portion of cold water is also required at the tap to give a right temperature mixing for showering or washing.
Particularly on old combi boilers, it isn’t always stable if you set hot water temperature at the boiler to 30% or so. It often gives hot and cold temperature cycling.
So the correct way is to set hot water at combi boilers to full or high, and get hot water mixed with cold water at the tap.