I agree with the rest... At 15% bf, you are in muscle. There are very few nerves in the glutes (and actually in any muscle), so the only time it hurts like a mofo is if you hit a nerve branch going through the muscle, or if you are too slow with the stab, and it cuts slowly through the skin and epidermis. A quick stab in the deep muscle should not hurt 90% of the time. I know of some doctors who now use 3" needles for penicillin and other injectibles, due to the obesity problem in the country. You'd have to be severely obese to need longer than a 1.5" needle. For most here, the opinion is that a 1" is enough, but I use a 1.5 to be extra-safe not to waste any gear by having it leak out into the sub-Q tissue.
Charles