I'm assuming this is an electrical desalter/dehydrator you are talking about.
You want the temperature as high as possible as that reduces the oil viscosity (which promotes separation) and the density difference between the oil and water. 300F is typically the upper operating limit because of the electrical connections to the vessel. My information is many years old so you may want to talk to someone like Natco for more up to date advise.
Once you set your maximum temperature the pressure follows because you don't want any vapor breakout in the vessel so it depends somewhat on your crude.