Fill the stove to the top of the firebrick. The firebrick is higher in the back, so you'll end up with a sloped coal bed.
I'm burning pea size in mine, I was not happy with the results with nut coal.
I generally run the draft vent anywhere from 3/4 turn to 2 1/2 if I need lots of heat. With nut size the draft adjustment will be different.
Go for the Field Controls RC damper, it's the best one. Go to their web site and you can find a distributor near you or take a ride up to Sussex County, PM me and I'll take you to the location that sells them.
After you install the damper cover it with foil and build a full, hot fire and see what the draft reads on the manometer. If it's over -.06" WC then you know where your heat and coal is going.