Not very scientific
I am obsessive about battery life when I get a new phone. For the first week or so I work out how long I can get from 100% down to 99% with light & heavy use, then down through every 10% point to zero. I can tell after a couple of charge cycles whether my battery is good after an iOS update or phone exchange.
For instance, with my standard morning usage pattern if I get over 45 mins on a 6S+ from 100-99% then it's good. If I get over 1h 50 from 100-90% then it's good...and so on. I save screenshots to guide me.
I can relate to being a little on the obsessive side with battery life and taking screenshots at different percentages too.
I usually do one at 50% then 20% then 5 or 10% then 1% sometimes if I'm going for a full drain. I like to see how much usage I have acquired at each of those percentages.
What I've noticed with iOS 9 compared to 8 is that when I lock my phone the usage number stops climbing immediately 99% of the time like it's supposed to. So when I come back and check say 30 minutes later it's the same as when I left it. On iOS 8 I've noticed in the same scenario that my usage minutes could go up 3-5 minutes or so after locking the phone like a background process was going or something for a few minutes. This would happen more often than not leading to unrealistic usage numbers meaning I wasn't actually using the phone as much as it was reporting and making me feel like I was having better battery life than I actually was! I often screenshot my usage before bed and check when I wake up and in an 8 hour period or so on iOS 8 my usage number could climb up 4-20 minutes (usually on the higher side) where as in iOS 9 the number will only climb 2-6 minutes every time so far all with the same settings. I find this number and behavior to more accurate in iOS 9. Therefore I've been getting about the same battery in iOS 9 but it's been reported as less but it is the true accurate number now!
I always check to see how long it takes for my battery to click off of 100% to 99 too and it's always been about 45 minutes now on every iPhone I've had and iOS version too as well. iPhone 4, 5 and even my 6 now. Same thing.
As for battery life on my iPhone 6 on 9.1 and every version before that I'm getting about 7 hours total usage of screen on time. That's if I'm just mostly using safari on wifi (brightness 50% or below) and tweetbot and light stuff like that. If I play a graphic intensive game for awhile goodbye battery! Maybe 2.5-3 hours screen on for a game. If I'm listening to a podcast through my headphones with the screen off in my pocket then sure I can get my usage numbers to look crazy and get 8-9 hours.
On a typical day of using the phone and browsing hard and messaging and mixing LTE and brightness going high some of the time I think 5:45-7 hours total usage is pretty typical with the 6. I'm talking mostly screen on time. I know the 6plus/6Splus battery absolutely smokes the 6/6s in battery but it's just too big for me. Don't you guys get like 9-10 hours of heave screen on usage times with those things?!? Anyways hope my experiences I've talked about here help someone!