Use the highest setting, which will put the most coal into the fire pot as long as it doesn't spill out the front door. Even then you could put some sort of bar across to prevent that.
The goal is to have the fire pot as full as possible for a long burn. The more coal there is burning, the longer the burn for a given heat output.
You will have to develop what system of clearing ash works best. How often will depend on how high the heat setting is and amount of coal burned. A simple shake might suffice in mild weather at low output. Experiment with shaking , slicing and poking to find what works best under varying conditions.