So I typically keep the top vent at about 1/3 and the bottom at 1/4. I try and do almost all of my adjustments from the bottom vent. If temps start to bottom out or spike I may adjust the top vent a small amount but I tap the bottom first to try and reign it in.
As far as amount of wood. I use the SnS on the opposite side of the vent loaded and lit their recommended way. I then add about 5 chunks across the top of the SnS, one on the lit part and the rest spread out across the unlit coals. For a 4 hour smoke before wrapping, I may add another chunck or two around hour 2.