Hi Doug, i wouldn't use darks with a 5 degree range eg:15 - 20 degrees too big of a gap. I use it per each degree. even with 1 degree difference, you'll notice it.
i normally do something like this:
19-20 degrees (any lights ranging from 20-19 eg: 19.5, 19.2, 19.8 degrees will be using these)
19-18 degrees (" " " ")
18-17 degrees (" " " ")