doing a user battery recalibration has in fact made my battery's reported available charge more reliable.
step 1: keep charging yr iPhone for at least 30 minutes after it reports 100%, then do a recalibration.
step 2: use naturally until the charge reads about 4% or 5% then do another recalibration.
step 3 recharge again until 100% and take it off charging
of course, most people in these forums say the above actually hurts yr battery. i swear by it.
I always do it, and it does have benefits. In fact, that reminds me that I need to calibrate the 17" MBP that I did last week.
I almost always use NewerTech batteries, and many of them have slightly higher capacities than the original Apple batteries. This makes calibration even more important as the computer won't necessarily accurately predict the percentage.
As a prime example, several months ago, I put a NewerTech battery in a 2012(I think) 13" Air. Through the calibration period, I watched the percentage reported by OS X vs. that reported by Coconut Battery. The system consistently reported the charge a couple of percent higher than Coconut Battery, and in fact it shut off with OS X still reporting about 3% remaining. After doing that, it reported accurately, or at least consistent with Coconut.
BTW, I don't know if I've seen a published procedure for recalibration with batteries that are not officially user replaceable, but I do it the way Apple says to do it for computers with removable batteries. Specifically, I fully charge it and let it sit on the charger for a few hours(generally overnight, although you don't need to leave it that long). I then use the computer until it shuts off, and let it sit at least 2 hours before plugging it in or otherwise attempting to get it to power back up. I've replaced quite a few non-replaceable MBP, MBA, and even MB(older) batteries and calibrated using that procedure.