I agree about the positioning of the button too. I have two guitars, an acoustic bass and a bouzouki. The bass and one guitar have the button on the back (flat part) of the heel and it doesn't work too well, as you have stated. The bouzouki and the other guitar are similar in set-up to your advice and work much better. I have suffered a couple of times from the strap coming adrift at the body and playing the rest of a song kneeling on the floor. I'll be getting the cigarette lighter fluid out next and singing Purple Haze! One sadly departed colleague suggested a special type of button which has a lock on it - he'd witnessed my embarrassment at the gig. I'm not sure if this works if you use a button with an inbuilt jack plug for a pickup - the button for these are huge and getting the strap on and keeping it there is not easy.