Unfortunately the gas gauge is not very good in the Gen II design. The best way to think of it is 2 pips is "get gas soon", 1 pip is "get gas now", and a flashing pip is "go to the nearest possible station and get gas!" Yes, there are people who've driven 50 miles on a flashing pip, but you don't want to be the one that gets 1 mile. The amount of gas per pip varies per car, and varies per tank of gas, hence the "guess gauge" nickname. However, if it runs out of gas and there are still two pips, there something wrong with the gauge and you should have it fixed.
The most I've ever pumped was 11.624 gals, but because gas is $.40 a gallon more expensive here in the mountains, I always top-off before I leave the flatlands, knowing I'll burn about 1.5 gallons on the way home.
About a month and a half ago, I fit 12.455 gallons in my tank, however I try to squeeze in as much fuel as I can when I fill up. One note about this situation, is it was a very hot july day, near 100F. My typical fuel up is 10.5-11.5 gallons.
Don't try to squeeze in as much as you can. Doing so saturates the vapor canister, which increases hydrocarbon emissions, and can eventually damage the fuel tank and its valving system. That's about a $1000 repair.