I'm not a medical professional of any kind and nothing I post should be considered medical advice. Always ask your doctor and follow his/her advice.
What do you mean by going in too far...you think if you penetrate too far that can also cause plunger resistance?
Yes. While I'm no expert on anatomy it has been
my experience that resistance can increase if you go too deep, although I'm not sure what structures you would be piercing to cause that result. There are times when I have to back out a bit. While I'm not generally a big advocate of aspirating, and I never look for or expect blood, I have at times found it helpful, when I've thought I was in the correct spot, to back out the plunger 1.5mm or so before pushing it down. This will help indicate if you're positioned correctly as it will be difficult to pull it back if incorrectly positioned. It also will free the plunger from it's initial resistance against the walls of the syringe, making it easier to depress, assuming of course you're in the right spot. I hope that train of thought wasn't too muddled.
Let's say you've got a flaccid penis length of 9cm, at what locations do you inject, 0cm being the base of the penis and 9cm the tip?
While I haven't seen it recommended in any medical literature or heard it from a doctor,
for me, I feel anywhere from 9mm from the base to 9mm from the glans is fair game although too close to either end may be more painful and needle depth may well vary when at either extreme. Near the glans I've found shallower is the ticket, while near the base deeper is needed. Of course it's easier when more conventionally located but after many sticks I find it helpful to be able to branch out a bit.
Good luck!