Personally, I like having it come on with 3 gallons left. I'd rather see that light and be given enough time to find a place to stop than have to sweat 16 miles hoping for a gas station. Come travel some backroads in Oklahoma and Arkansas with me -- 1 to 1.5 gallons won't get you to a gas station. When I used to get 26 miles to the gallon on a car, 1 to 1.5 miles was an adequate warning. But at 15 to 16 miles to the gallon, I'll take that extra margin of comfort.
That being said, I don't trust a light to tell me when I need gas -- I use the gauge and my mileage. As long as I don't rely on it, it can't cry wolf. In other words, when it comes on it gets my attention. I've been stranded three times too many by an ex-wife that absolutely refused to use the gauge and relied on the light. I have a 1/4 tank rule... when it hits 1/4 tank, start thinking about gas. When I hit an 1/8th of a tank I get nervous -- it doesn't matter what I'm driving. Anyway, 3 gallons out of 19 is about 16% remaining, which is a hair over an 1/8th of a tank... which is right where my light comes on.
My gauge is pretty accurate. If yours is bugging you, have it checked out. I have trouble believing DC goes out of their way to spec crappy gauges for a single model of their entire product line.
2004 Wrangler Unlimited, Shale Green, soft-top, Shrockwork Rocker Guards, Kumho 31x10.5 AT's
Green Jeep Club Member #119