Duncan, the last ones i did i just wrapped them in foil and let them on there for about 2-2.5 hours.(top Rack) You just have to kepp checking them to feel when their done I was cooking at 240 dome temp.
I don't think I would waste precious space on baked potatos. I would foil them and put then in a kettle or gasser. You're not going to get any smoke ring on a potato. I would find an extra piece of meat somewhere and put it on the extra rack or stick on the end of one of those spikes.
Click "Find" and do a search on "potatoes". In the results you'll find three good discussion threads about smoked potatoes by Bryan S., Kevin Taylor, and Rita Y.