I copied this from a post I made in a thread from 2010 that got some positive feedback. I know you know this already, but here it is:
Here's a gif of Rosewall's driven slice:
Check the height of his hitting hand from takeback to contact to followthrough. While he demonstrates the classic "U" shaped swing for slice his hand doesn't stray too far above the intended contact point in either the take back or the followthrough. IOW it's a very flat U shaped swing. In an attempt to impart backspin, many players employ an exaggeratedly high take back with their playing hand above shoulder height, even though they are going to make contact at about hip level. That high a takeback sets up for a much more severe downward path of the forward swing, describing nearer a right angle between the path of the ball and the path of the swing. That severe an angle creates a very narrow window requiring exceptional timing for clean contact. The faster the incoming ball the more precise the timing will have to be with a very severe downward motion. Again look at how compact and quiet Rosewall's stroke is, look at the hitting hand, not the racket head. Note how "flat" Rosewall's U-shaped path is.
This link draws a distintion between the example of Rosewall vs. Federer. Compare the flatness of Rosewall's playing hand to Federer's "relative chopping motion" and to yours.
2) Dr. Jack Groppel, PhD, a tennis coach who bases alot of his coaching on science states in "High Tech Tennis" that if the trajectory or ball flight from the strings to the bounce hit with underspin strikes the court surface at an angle of 45 degrees or greater, the rebound will tend to be greater than that and that in inverse is true at incoming angles less than 45.
For example a ball which impacts the court at 55 degrees will rebound at an angle of 65 degrees, where everything else being equal a ball impacting the court at 30 degrees will rebound at 28 degrees.
<edit> also, be mindful of how open the racket face is on contact, no matter how "flat" the U-shaped swing path is if the racket face is too open (facing the sky) that player's slice will tend to float and sit up. While not purely square (90 degrees to the court surface) it should be closer to 90 than 45 or less, depending on the amount of pace applied to a given shot.