Please allow me to give you my opinion on this as I love running with the tank almost empty :In fact, I don't like to fill up, that is the real reason.
When the caution light comes on, I will say you have about 8 liters (2 gallons to be conservative) in the tank left.For me, it means that I can still go about 100kms.
From what I learned from the salesperson (at least, he was informative on this part as he owns a Versa), when the last square disappears, run to the nearest gas station (only 10kms left).
As several have seen, and it is the case in every car, no just nissans, the first part of the gauge goes slower than the last part. So when your gauge is at about less than two third, you are in fact at mid tank.But, the last squares are the most important, and from what I have seen, 1 square = about 50 kms on my car.
I always use my partial odometer (reset after every fill, A then B then A) to know how far I am and how many kms I can still go because the gas gauge is too imprecise to be sure. Sometimes I gain a square back and then lose two in a row, acting kinda weird some time to time depending of the weather (heat and cold play on the gas being in liquid phase or gaseous which play on the gauge).
As I experimented myself, I filled my car 68 liters once (No typing error here). I did not have any square left and drive about 10kms before filling.
On my previous cars, I had already put more fuel than said in the manual but usually it is no more than 2 to 3 liters (because of the pipes on one hand and because I fill the tank until I can see the gas (about 4 liters after the automatic stop) on the other hand).
So, I believe I have a 65l (17 gallons) tank which is better than a 60l tank for me .
I have just filled up the car yesterday: 66 liters with still a square left.Definitively not a 60l tank on mine!
If you need a cobaye for stupid experimentations on the car, ask me, I am doing that on a regular basis