I generally keep 2-3 vents open on mine. My vents are 3/4" nipples, so perhaps that is the reason.
It sounds like you've got a handle on it, but if you have good airflow around your basket, your drum is sealed, and your basket is large enough, you should be able to go 14-24 hours with a UDS at a very steady temp (225-250 in my case). I have, however, had better luck with certain fuels. For long burns, I have had the best results with a good charcoal. Now I haven't tried all the brands, but the Kingsford competition briquettes seem to give me very stable temps, little ash, and burn forever. My experience has been that lump, as much as I like it, is not as consistent.
Like N8man said in his tutorial, there is no wasted fuel in a UDS (as long as it is sealed), so load that thing up. I'll put 20 lbs in mine, just shut it off when done.... burn the rest next time.
UDS, 30 Year Old Kettle, I need to build something bigger. KCBS CBJ