The Hotblast stoves usually need a lot of draft to burn well,, so I'd leave the Manual damper open untill you get some experience with the stove. If you are getting 12 hour burns with it, you are doing well..
If you want to fine tune the furnace, you need to get a manometer to measure the chimney draft, and a thermometer or two to measure what is going on in the stove and in the flue.. otherwise you are pretty much guessing at what happens when you make a change, and wait to see if there is any difference in heat or draft..
I assume you are filling the firebox as full as possible, and you need to close off any air vents above the fire, use only the air vent in the ashpan door. If you are able to get the fire to burn for 12 hours, you are doing well.
Once you get a draft guage, [manometer] you can set your baro so that you have a max draft of about .07", and then you can monitor your temps with a few thermometers..