The "miles to empty" calculation is done by looking at your overall mpg, and how full the car thinks its tank is. If you get 40 mpg and have 10 gallons left, then 400 miles to empty. But if you get 22 mpg ordinarily, then that same 10 gallon tank will read 220 miles to empty.
I say "the car thinks" because it's become increasingly clear to me that the computer doesn't have a sensor that tells it how much liquid is actually in the tank, nor does it seem to have one that monitors the flow through the pump. It's using some other information to make its guesses, which means that running significant amounts of ethanol really screws up its estimates, since the energy density is wildly different from what it expects.
If you're using Ultragauge there are ways to program those parameters, so you can get more accurate numbers... but it doesn't use those sensors either (probably because as far as I can tell, they just don't exist). Torque, you could probably install sensors and program it to read those PIDs and do the actual math.
It will never be more than an estimate though, because you might end up driving up a mountain in a headwind at 85, cutting your mpg to a fraction of what it normally is... and not get anywhere close to your DTE.