I think the standard instructions for most is to inject at the 2 or 10 position, halfway or closer to the body. Considered as a whole, my doc says to inject about
3/4 back from the tip of the penis, which will place it in the cavity easily. Yes, the skin on mine does tighten, but only because the erections are much larger than the flaccid penis, so it's just the nature of the beast. Filling and lengthening will do that. Of more concern for me is the head doesn't match the shaft in hardness.