There are multiple issues that make each UDS differennt on how much they must be chocked to maintain temperature. Temperature is controlled by draft. Draft is created through your intake holes, but has no problem creating itself through cracks and the exhaust. Your exhaust holes are probably allowing slightly more draft than a smoke stack system, so I would not really worry that you need to choke at the ball valve with both nipples closed.
fwiw fuel also effects this. Damp lump takes a lot more draft to hit temps than good dry lump. I suspect that draft requirements also change between lump and briquettes.