If you are worrying about being out of gas when the "idiot light" comes on, I suggest that light is just a heads up. There is a LOT of variation in when that light comes on across the HD lines, and even within a model/year. I would suggest that light is a heads up to make you look at the trip odometer and figure how many more miles/kms you have to go before you are pushing the beast. Record your fuel purchased vs. mile/kms traveled for a few fill ups. This gives you a reading on your average fuel economy. Use that to figure your MAXIMUM miles on a tank given the fuel capacity of your model (probably 5 gals.). Then use the light as a reminder to double check where you are, calculate quickly how many mile to safety (maximum miles - 20), and then start looking for gas around that point.
If you make yourself a simple little spreadsheet and record your fuel purchased and distance traveled religiously, you will know when to worry.
See attached.
TQ