When doing your one-legged squats, you want to simulate the sit spin position, which means having your heel lifted (remember, when you are spinning, not only are you wearing a skate with a heel, but you are lifting that heel off the ice, too). So don't do them in flat shoes; do them in shoes with at least 2" heels (3" is great). Not only will that put you on the ball of your foot and better simulate the alignment you need in a sit spin, but it will also be easier and won't be so hard on your knee. Try to get your back arched and chest forward, with the inside of the thigh of your free leg pressing against the calf of your squatting leg. After doing that off ice in front of the mirror, try doing the same thing in a backward (not forward) shoot-the-duck position on the ice, with your heel lifted so you are on the ball of the blade just behind the toepick. Those are the two exercises that got me my sit spin position.