Remaining time on a battery is at the current amount of drain drain (at the the precise moment the calculation is being made). To determine, within even 1%, exactly how much time REALLY remains in the battery would require a method of seeing into the future. Unless your phone is in airplane mode and doing exactly the same thing from now until the battery dies, the best estimate of remaining battery life is probably on the order of 5-10%. Even 1% is just making things up. 1/1,000%? That's beyond fantasy. It's like predicting the lifespan of the third child of your second grandchild when you're 5 years old.
If the phone is not in airplane mode, moving one joint of one finger that's 3 feet from the phone affects the drain on the battery. So does an airplane passing overhead or a car passing on the next block. And you want accuracy from that? We can account for the butterfly effect in weather prediction a lot better than we can predict cellphone battery life.