The Back Stance (also sometimes called an L Stance or dwi kubi seogi) is performed with the leading foot pointed directly forward, the rear foot perpendicular at a 90 degree angle from the front foot. (This is why it it sometimes called an L Stance, your two feet are placed like the letter L. See also, Left Stance and Right Stance.)
The front foot is about one stride (three foot lengths) in front of the rear foot, so that the heel of the front foot points back directly to the heel of the rear foot. Most of the weight (about two thirds) should be on the rear leg. Both legs are bent at the knees, and both feet are flat on the floor.