Another point. You can do as many SDC as you want during the day. I have often rolled through several flights later in the day to make sure I was on something.

As to the LAS-ATL flight, a word of caution. In your case there are no other later flights and I've seen flights that were only 2/3's full in the afternoon, sell out by the time the red eye takes off..... Make sure you call at exactly 3 hours prior