I have found, through trial and error, that if I put just enough wood in my boiler so that when I return to fill it next time there are very few unburned pieces of any size, I burn less wood and just as importantly there is way less ash build up. All I have to do now, as others have stated, is a 20 second rake job on the bed of coals, and chuck in enough wood to get me to the next fill time.
When I first started, I was putting in way too much wood and too often... my ashes (more like unburnt nuggets) would build up rather quickly. Now, my ash is burning to a fine powder and is reduced to at least 1/4 the volume.
You would be surprised how long the water will stay up to temp as your stove burns the coals into fine ash.
The only time that I "over fill" is if I know I will be out of my normal fill routine (home later than usual for whatever reason).
Although I don't own a Hawken, I'm sure it is the same for most conventional OWB's.
Although I don't own a Hawken, I did stay at a Holiday Inn Express last night