For the Focus, when it gets that low I PLAN on only having 1 gal. of "useable" fuel, and really haven't bothered to push it to see EXACTLY how far I could go.
Done that before by carrying a 5 gal. can to fire one up again when it dies just to see what is ACTUALLY useable, but it is a LOT of work to plan that experiment so you don't get stuck in a bad place when it dies....
Fuel injection has the annoying habit of dying COMPLETELY when it runs out, and needing quite a bit of fuel & time to get running again. totally unlike a carbureted vehicle that you can always get a LITTLE more out of when it "sputters..