Proper Draft Control Usage (Severn Boiler)

I have a 1957 Severn boiler. I plan on burning wood and coal in it.
There is a draft control on the back that opens into the ash pit and feeds combustion air under the fire. There is also a draft control in the loading door that feeds air over the fire.
Can someone detail the proper usage of these two air feeds when using wood and when using coal to maximize burn time with each fuel.

I may have come across the answer to the question of which draft is best to use when burning wood on this website.
http://www.charmaster.com/exclusive_draft.html