Estimating remaining energy/distance

I find the "energy remaining" bars on the console hard to use to estimate how much time/distance I have left. It's pretty quantized. Plus it seems unreliable. On some rides on windless days and flat ground (using assist level 2), it'll show a third gone 10 miles into a ride, and other days, under identical conditions, it'll show a half gone. But in both cases I usually end up with nearly the same distance by the time I'm down the the last 1/2 bar. Typically at 36 miles.

Some of this variation seems to be whether I've recently used up most of the energy in the battery. A full recharge, instead of the partial recharges I normally do, seems to get it back on track.

I have the "distance remaining" option turned on. But that just uses the last couple of seconds of use to estimate how much further I can go. That and the "time remaining" functions are useful to figure out the rate you are using power, but not how far you can really go.

What I'd really like is to be able to check, about 1/3rd into a long ride, how much energy I really have left so I can figure how much farther I can go, assuming the same conditions.

I've thought of looking at the voltage on the battery to do this. Assuming a linear relationship between voltage and remaining energy, and knowledge of the charged and discharged voltages, this would be easy. But I'm assuming that's what the bar display is already doing.

Any ideas?