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.