All depends on where the water gauge sits. You certainly dont want to over fill a boiler as you will get hot water instead of steam. My advice is that you must have the flue covered assuming its a horizontal boiler, then as little water as possible to satisfy your time in steam. This is the mose efficient way of steaming and means that you are not heating water which will not be used. For a vertical boiler my rule of thumb is to have 1/3 water left at the end. This ensures the boiler and flue dont overheat.