Hi Augor, I have read a manual, with pictures, that explained it thusly....
Assuming quad is parallel to floor (i.e. sitting on a chair with foot flat on the floor), injection height in relation to the leg, is about 1/2 inch above the bone, into the quad, for about another 1 1/2 inches. In relation to position between hip and knee, place the heel of your hand at your hip with your fingers pointing forward along the quad. Where the fingers stop, this would be the upper most region where the injection site stops. Similarly, place your fingers at the knee bend, again with your hand running along the quad, where the heel of your hand stops, that is the lower most region of the injection site. So, now you have a pretty large area within which to pin which will bring down the chances of generating scar tissue...
Does that sound like something that you have heard?
Good luck,
H