It's best to set your TRV to a middle setting and see if the room is warm enough. If the room is too warm, turn the TRV down by half a mark, if the room is too cold, turn the TRV up by half a point. A middle setting corresponds to approximately 20ºC (68F).
e.g. If your TRV is set at 3, and the room is too cold, turn the valve up to 3 ½.
If the room was too warm, turn it down to 2 ½
Do NOT turn the TRV fully ON or fully OFF to make the room you are in warmer or cooler - this will waste fuel. The most effective way is to turn the valve a little way and let it do the work.