To look at the first 7 days:
Toy Story 3 versus Despicable Me 2: both animated, close release dates, not that many years between them, and close on the 7 day totals. TS3 did 110.3m Fri-Sun while DM2 was at 83.5m, 25% lower (likely boosted somewhat by having a holiday close to the weekend).
Another might be Transformers 2 compared to TDKR and Deathly Hallows 2. All summer openings, first 7 days are close, but the Friday openers were in the 160-170 range while TF was at 109 for it's Fri-Sun, 32-35% down.
So obviously I'm mentioning this in reference to guessing at the hobbit 3's opening, which leads me here:
Given those percentages, a wednesday opening at Smaug's level would be around 50 I think for Fri-Sun, and at AUJ's level it would be up to low 60's at most, around Two Towers level. Not that it can't be bigger, but if you're expecting it to open similarly to the last two you probably aren't thinking 70m for Fri-Sun, right?
There's probably a more sophisticated way to go about it but that's all I got for now.