I'll have to look when I get home, but I think the bottom of the tank is nearly flat at the rear, so there would probably only be a few ounces of unusable gas there. The reserve on the older Brit bikes was that you only open one petcock at a time. I remember from the starting instruction book on my 650 Triumph, "Open petcock on the pretol tank, the right one by preference..." Then, when you run out, you open the left one for a reserve.
