looking at your back, armpit is when you're hair is in line with the top of the crease of your armpit (with your arms down). Because i think brastrap can vary depending on what bra your wearing or how your wearing it, I consider brastrap as your hair being inline with where the wire would be in an underwire bra (or in other words, where the underpart of your boob connects to your body), so I guess the bottom seam of your bra strap. But that's just me.