There are so many theories out there on this topic…
* Hit the head ball straight on with a stop shot position at your break speed.
* Draw a line through all three outer balls (depending on side you break from) and hit the center of the line with a stop shot.
* Etc…
But each of the theories does not take into account your individual stroke. Your stroke may have a tighter grip or you may not have a loose wrist just like the person who wrote that section. Their centerline stop shot may be dead center while you are off slightly to one side or the other (even 1mm can make a difference).
The answer lies in the practice of the break stroke and playing with your break speed until you can predict the final position of the cue and the 1 ball so you are set up for your first object ball after the break.
Practicing the break is a time consuming exercise but there is a tool out there that allows you to work on your mechanics while you search for your personal combination of what will work for you. The tool is Break Rack Practice Tool found at the following location:
http://www.breakrak.com/
I have tried the product through one of my league friends and it is a great so that you can work on either the power, position of the cue after the break or just your break stroke mechanics. But practice is the key to find your best break.
Sorry if I did not address your immediate question on where to hit but there are just too many variables.
Good luck and enjoy the game...
Alpine9430