Personally, I wouldn't worry too much. In an ideal caging situation, you want a temperature gradient (a low end and a high end)so your snake can choose where it is most comfortable - so it can thermoregulate. It sounds like you have that fairly well covered.
From what I have read here, it sounds like Dumerils like temps a bit lower than most would think (as in not in the high 80's or 90's) - other people would probably have more specific info on this than I do. I'm sorry that I can't be of more help here; I currently own only one Dumeril's Boa, though I have kept over a hundred snakes of various species - so I'm good with the general info.
I completely understand your fears though, too. Without becoming a hypochondriac keeper, watch your snake to be sure that it is not showing any signs of a respiratory infection - swelling in the throat or mouth, bubbly clear mucus present in or around the mouth or nostrils, etc. Other than that I think it is fair to trust your snake to make itself comfortable.
If you don't have a basking lamp, that may be a nice addition - but be careful! Adding a basking lamp over or near your existing heater may make the cage a lot hotter than you intend. Measure the temps in all locations in the cage (as it is), and then measure them again if you decide to add anything.
Is this your first snake? How big is this cage?
I hope this helps. 
~Rebecca
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1.1 Ball Pythons (1.0 '05 Ghost, 0.1 '03 Normal)
0.1 Dumeril's Boa '04
0.2 American Pit Bull Terriers (40 lb darling lap dogs)