Oh,
BOY, have I noticed this issue. Talk about "Your mileage may vary", right?

I stopped paying attention to that alert after I had been registered on Fuelly.com for awhile and was able to get a good sample group of what the car's range was based on my driving habits (
which are actually pretty conservative considering the engine mods that have been added) and the amount of driving I do each day.
I've found that by the time the display is completely blank (i.e., no "lines" show) I've got MAYBE another day's worth of driving I can do. If I think I'm running too close for comfort I just wait until I've been driving for 450 miles since my last fill-up and re-fuel then.
Disclaimer: I ran completely out of gas yesterday morning (within a few hundred yards of a gas station) for the first time doing this, but my driving habits during that tankful were not typical and I had ranged out at 458 miles by then.
Again this is me and my driving style; your results will probably be different.