The fire is adjusted based on the size of the ash ring, not how high it is. But since you asked...when the stoker is working hard the fire should be slightly higher (maybe an inch?) than the top of the pot...not sunken in the center.
When the stoker has been running for at least 30-40 minutes without stopping, the fire should have about 2" of ash around it. The reason for this is that you don't want to blow air through ash, you want to blow it through coal that is burning. Air that is blown through ash just steals heat from the boiler on its way to the chimney.
To put it another way, when the stoker has run long enough for the fire to stabilize, you want the line between fire and ash to be along the inside edge of the pot ring.