Most 'accurate' gauge I ever had was on a Stratoliner,,, It had 2 fuel tanks,,, Light would come on when main was empty and the '2nd tank' (located down and back from main, with the fuel pump in it) was in use. Had about 7-8/10th of a gallon then,,, Also had a countdown on the odometer starting when the light came on,,, pretty slick, actually.
The low fuel light (No gauge) on my old VTX18C was a bit funky,,, It was temperature sensitive,,, would start to glow ever so faintly, and then brighten to full on. ("Sloshing' the gas would make it go out if it was just 'glowing') Kind of a guessing game with it daylight vs dark, how often you looked, etc)
The light on the 6 is no exact science,,, Wifes light comes on a good distance before mine,,, She also has more gas left in her tank at that time than I do, so
Seems to be consistent in it's warning, but not necessarily consistent between bikes,,,