Unfortunately, battery apps simply aren't reliable in reporting the health of your older iPhone's battery. They might give you a general idea that your battery has degraded since the day you bought it, but I've seen wildly varying percentage numbers regarding my iPhone 6S Plus' battery health.
The best way to find out the health of your iPhone's battery is to take it to an Apple store. Set up an appointment and an Apple Genius will run your iPhone through the company's series of tests.
Usually, Apple recommends that you get a battery replacement when your iPhone's battery can only retain 80% of its original capacity. With Apple's new discounted $29 battery replacement program in mind, however, I'd personally get it replaced even at 90% capacity.
You can get in touch with Apple Support remotely over the phone or even Twitter to find out whether you need a batter replacement or not, but they won't tell you specific details about your iPhone's battery health. The company's support teams will only tell you whether your battery is "fine" or not.
In its statement regarding the performance throttling controversy, Apple said it will also issue an iOS software update early this year "with new features that give users more visibility into the health of their iPhone’s battery, so they can see for themselves if its condition is affecting performance."