My backhand slice works really well so I figure I can help you out a bit.
First of all make sure you're using a eastern - continental grip for your slice, eastern is almost too far (I use continental)
What you do is angle your string bed so it's almost pointing up to the sky, use a full unit turn and bring your racquet behind your shoulder.
Now as it comes in front of you, you slice into the ball with the angled racquet and drag the ball across the strings, you'll feel the difference and the longer its on the stringbed/faster you swing the more spin you'll get (due to the more cut.)
My slices clear the net in window 1 always, thats roughly below 2 feet, thats a good target. If you're inside the baseline you can let it clear much lower i.e a few inches above the cord. These are great shots because the slice makes it kick in strange directions sometimes, depending how you hit the ball and it stays very low.
If you're behind the baseline and stretching for a shot where you can't use a twohand backhand and use a slice to defend, this is where you want the height. A high slice, i.e window 2 will allow you the time to get back into position and recover, it will also be a good change because is a slower ball, meaning you're varying the pace, along with buying some time to get back into the point.
Just watch out for people who are approaching when you hit it, really try to keep the slice down then, there's not much easier to volley then a high slice coming at you shoulder level when you're at the net