Humidity is not a problem as I live in the desert and own a smokin it smoker which tends keep my cuts from drying out. Thanks.
Even with common temps, cook times vary from meat to meat, so it's all an estimate.
With my new pellet smoker, I've begun starting out at a lower temp for the first 1/3 to 1/2 or the cook to get a good smoke, and then increasing the temp to help reduce overall time.
Aside from that, all that I can offer is to experiment.
I can't speak for Rib, per se, but for Brisket and Butts I have found you can "estimate" a 30 minute per pound change moving the temp up or down 25°F. The average Pork Butt will go 2 hrs/Lb at 225-250°F, 1.5 hrs/Lb at 250-275°F, etc. If you drop down to 200°F, I would figure 2.5 hrs/Lb for a Butt. I would plan on an extra 30-60 minutes cook time for Ribs...JJ
40 to 140 in 4 Danger Zone Time only is an issue with meats that have had their surface compromised. Briskets that have been injected, Butts that folks punch holes for Garlic and any Enhanced meat like Turkey. If you don't inject the Ribs, there is nothing to worry about. Lastly, with the exception of Beef that will be served Med/Rare, Refer directly to Smoker is the safest method to smoke it...JJ