... covering numerous angles of break shots and exactly how to hit them. ...
By now you should have a library. Review what's in Mosconi, Byrne, Fels, Capelle, Cranfield. Capelle has a book called "Break Shot Patterns" which may help you get a good angle more often on the break ball.
Experiment. I had a particular break shot that stopped a run. I got out some donuts and started practicing that exact break shot. (It was a 45-degree cut on the wrong side of the rack, so I was shooting from the end rail). What I discovered with that shot is that for that exact shot I could not predict which ball in the rack I would hit. If the ball went in on the left side of the pocket, I would hit the nearer ball in the rack and the cue ball would get loose even for a nice, controlled speed. If the ball went in on the right side of the pocket, the cue ball would hit the farther ball in the rack and stick -- a very nice safety.
One point of relating that experiment is that there is no precise rule. There are guidelines, though. Also, you need to experiment. If you are working on the best way to get free on a behind-the-rack break shot, get out the donuts and experiment with slightly different positions -- move the donut a quarter-inch at a time to see the different reactions off the rack for shots that look identical from a distance.
One thing I've been doing lately is picking the ball in the rack I intend to hit and focusing on just that ball and the object ball and visualizing the carom from the object ball to the rack ball.
General ideas:
0. Get the cue ball free.
1. A ball that is closer to the rack is better than a distant one.
2. A thin cut is better than a thick cut.
3. The cue ball should not leave the bottom half of the table.
4. If you have a problem with accuracy on power shots, see if finesse works for the problem position.
5. Others?
As an example of 4: In the old days, when a player was faced with Hohmann's standard break position -- side of the rack break with the cue ball straight up the table and maybe even with or above the side pockets -- the absolutely standard play was a soft follow shot that brought a couple of balls out of the back of the rack and left the cue ball near the foot rail for a secondary break shot. If someone had drawn the cue ball to the head rail and back, his friends would have taken him in for evaluation.
I think it is very useful to shoot the same shot multiple times. If you are doing high-run practice, start each turn with the same break shot that you want to work on. (Or just do the break shot, or just run the rack, or just make the first five balls, or...) Doing the same one multiple times lets you learn what can happen (like my sticking ball above) and what it takes to have a second shot.