You're asking for trouble if you accept someone's "after final bar" range to be the same as yours. These gas gauges are anything but precison instruments and there is tremendous bike-to-bike variability.
Your best bet it to take it as low as you dare, noting how the gauge behaves, then fill it up to the brim. The amount you put in as compared to full capacity (probably 5.8 gallons) tells you how much you had left. For this, you should be able to infer pretty closely what it had at the last segment in the gas gauge. But even here you need to be somewhat conservative since advertised tank capacity and actual capacity may not be exactly the same and also may vary from bike to bike.
To really put a fine point on it, you probably need to carry some gas and run it to empty. But you'd really have to be into stretching the last drop to make this effort worth the trouble.