I've been reluctant to post this... as I figured I'd be vilified... but the mileage meter in the panel works quite well for me. My wife also understood what it was doing after a couple of sentences of explanation. She's been happy with it ever since*.
Leave the house with a big number; get on the highway and drive 75 or 80, it drops very quickly and then stabilizes. Obviously, that's how far you can go at 75 or 80. Exit the highway to surface streets and drive 35 or 40 and it will go up somewhat slowly. Obviously, it doesn't want to strand you by being too optimistic too quickly... but after a while, it goes back up and that's how far you can go when driving at 35 or 40. And so on.
In short, it works really well for us. And, to the original question, yes, I live in flatland. North Texas to be precise. Less than 100 feet elevation change for hundreds of miles around (with the exception of river or creek valleys, which we tend to bridge).
I am an absolute techno-geek and may build my own SOC meter, and/or track other messages on the CAN buss(es). But this is my inner geek. As a driver, I am quite happy with provided mile meter.
* In fact, it saved her the other day. She had a round trip that was going to be close to total range. She set off on at her usual 80+ MPH. As the miles plummeted, she realized she'd have to slow down a bit to make it. She slowed to 65 and ignored the meter for a few minutes. As it came up to a more sane estimate, she settled on that speed, and made her trip at 65 with no sweat. If she had maintained 80 all the way, I'm pretty sure she would have come up short. She arrived back home in "low" (but not "very low") warning state.
I am unclear if Gary's SOC meter would have saved her.