That's what "reserve" is for. It's just like a low fuel light.
My magna doesn't have a gas gauge, nor does my KLR650.
My CBR has one, though, and it still has reserve.

I either know by the mileage when I need to fill up, or I wait till the bike starts sputtering and then switch to reserve and find a station.

 

this is a fact... your lost if your not paying attention to the basics(milage per tank)