Did you know that there actually is a rule on this that the refs in Vegas use, and we have adopted it in our region as well? Here is how that one goes:
If the cueball is closer than the width of a piece of chalk to the object ball, there is a high likelihood that a double hit will occur. If you jack up 45 degrees (approx) or shoot the ball at an angle, it WILL NOT be a foul. If you don't do one of these things, and the cueball moves forward, it will be a foul. If the cueball is frozen to the object ball, you can do whatever you want...including shoot straight through.
I know there are many examples of how you could jack up and still foul, or not jack up and not foul. The rule as stated makes it super simple, and very easy to enforce. I would say that after the year we have been using it in our area, it works fine. It has definitely cut way down on those shots where the cueball takes off like a rocket after the object ball which is a blatant double hit.
They ought to add something about this to the rule book too.
KMRUNOUT