I don't think the air temps matter as much as the substrate temps. Air temps tend to be pretty standard, even in Nature. I am new to this too, but I think 81 at night on the "cool side" may be a little high- but then I have also heard of people keeping their snakes all at one temperature, say, in a heated room, so maybe it doesn't matter as much.
To keep a "warm side" and a "cool side" I bought a smaller heat mat then reccomended, and I placed it to the extreme of my 20 long. This gives me 84.1 degrees at the very surface of the 2" aspen bed. The other side is mid-high 70's, but that's at least 12" from the heat mat. She have not ventured to that side voluntarily- at least when I have seen.