Answer:
You have seven layers of skin, and you want to go about half of that depth for your best result. You are injecting the pigment into the dermis layer. Going too deep will result in blown and blurry lines and going too shallow will guarantee that the pigment over time will be broken down much quicker than it should be and the tattoo will fade much quicker. Also, when tattoos are done too shallow, a lot of the pigment will "fall out" during the healing.