Well...congratulations on the fresh install! Nice work. After a fresh install I had the same experience initially...the battery life estimate was pretty unpredictable. It took a good 3 to 5 charge cycles before things settled in. Even then another two or three cycles after that before it really began to be quite accurate.
So I'd give it a while. Charge it up fully, leaving it plugged in for a couple of hours after the MagSafe connector LED turns green and then discharge down to something under 40% before plugging it back in again. Follow that routine a half-dozen times and you should see some reliability in however it's going to behave.
Also keep in mind that after a fresh install Spotlight will be indexing your entire hard drive for a while (plus more indexing when you reinstall apps, add files, etc.). Indexing takes a lot of CPU/power. It will also slow the machine down some. So again, give it a while to stabilize.
I can't recall which model MacBook you have (2012 MacBook Pro?), but I believe the spec is around 7 hours of WiFi web browsing (check here). That's light duty, so any heavy lifting (playing videos, etc.) will be less. Somewhere around 5 to 6+ hours of mixed use seems to be about average...again...give it some time to get there though. If that turns out not to be the case, then I'd certainly head back to the Apple store.
Congrats again and best of luck!