Hi Chris, I would try to keep the starter between 75-80F for this stage of development, but once your starter is rising predictably, anything in the 70-80F range is fine. I also wouldn't worry too much about humidity for the starter environment, although if you find that your starter seems thicker than normal, it won't hurt to add a little extra water when you do a feeding. Look for the consistency of the starter to be rather paste-like (but still easy to stir) right after a feeding, but as it begins rising and fermenting it should thin out and become more like a thick pancake batter. It is important , however, to cover the starter securely enough so that the surface doesn't dry out. The reason we say to "cover loosely" isn't because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can collect in a tightly lidded container and cause the lid to pop off. As long as you leave some space above the starter for the gases to collect, and open the jar now and then, it's fine to use a lid or cover your container securely with plastic wrap.

It can sometimes be difficult to see the starter doubling as we describe if you have it stored in a wide bowl or Tupperware container. For the quantity of starter recommended in our recipe, a wide mouth quart-sized canning jar that is taller than it is wide works well. If you're worried about going through too much flour during the creation process you could also consider maintaining a smaller starter, at least until your starter begins doubling predictably. Our smaller starter recipe fits nicely in an 8-ounce canning jar. 

The fact that your starter is rising is a good sign, so I would encourage you to continue. In order to promote more vigorous yeast activity, it's helpful to feed the starter before it falls, so try to pay attention to your starter's rising behavior and replenish your starter when it is ripe (at its peak of rising, or just beginning to fall). It's possible that the warmer conditions in your oven were making the ripening process happen a little faster than is ideal, which could have contributed to its current sluggish behavior. 

I hope this helps! Let us know how it goes. 

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