I've read and applied the following: Place you hand from the alternate side of your body flat on your chest with your pinky finger on your nipple, you may pin where the tip of your index finger lies.
Now this is just kind of a simple and unprofessional rule of thumb, but it worked for me. However, I was using 1/2" insuling needles (IGF-mfg). From my knowlegde, Ive been told not to go more than an inch into your chest (Surely if you have a chest like Big Ron it wouldn't matter, but that isn't normally the case). As long as you aspirate I do not see any veins or arteries being and issue. Especially if you're only goin 1/2". Be sure to inject very slowly, bc I'm sure lumps on the chest would be a bitch.