I tried the following today, and it turned out to be fairly accurate.

We know the 2004 TL has a 17 gallon tank, and the gas empty light comes on with about 3-4 gallons to go. Last time I waited until the trip computer said 0 mile left, and I filled the tank to almost exactly 14 gallons.

The trip computer gives you fuel economy and distance traveled (assuming you reset everytime you fill up). Today, before I started the gas pump, I quickly calculated the following:

Total distance: 340
MPG: 22

340/22 = 15.45 gallons (using the calculator built into the navi!)

When I topped the tank, it came out to 15.47 gallons

Subtract from 17 gallons, you'll have some idea how much gas is actually left in the tank.

 

I prefer the keep driving till there's no more gas method, but I guess this one is a bit more sensible... Thanks for the tips!