Best helpful suggestion I can come up with is to check the fare-buckets on the days/times you want to travel, and see what the patterns look like a few days/weeks out, for the days-of-the-week you want to travel. Yet that's not ATL-SFO specific, that could apply to any route you're attempting to put the odds in your favor, in regards to an EUA.

I can't speak specifically on ATL-SFO, yet SFO in general appears to be a nightmare to get anything free for (unless a lucky Plat, or a high-fare Y). SFO is legendary as a nightmare on NW flights in regards to EUA, and I'm hearing that the UA forum (SFO being a UA hub?) considers this a tough market to use upgrade instruments.