I remember doing the maths in 7th gen when the reserve light turned on the car had good 8-10 liters of fuel in it.
My guess is that the last bar works as the reserve light.
But things can be different.
For instance in second generation vitz first light goes up to 70-80kms.
But in third generation first light goes of at 50-60km then at when the last bar is left it keepseems stable for 10-20 kms then starts blinking.
The best option is to get your car tank full at reserve light. Total fuel is 3-4 liters plus the fuel tank size.