Don - my thumbstrap is centered just above my #5 button and angled upwards a little. That's about where I've seen it on every accordion I've played. As far as if it's the outside edge or the flat bottom of the thumb that goes against the edge of the board, for most of the buttons, I have somewhere in between the edge and the bottom of my thumb against the edge of the treble board. But, that changes a little depending on where you are playing on the board. I actually had this same question when I started and Jude Moreau told me, "It doesn't matter, just don't put your thumb underneath the board"
Oh and Don, as that thumb strap starts to stretch, or you just can't seem to get that tight, but not too tight feeling around your thumb even after you've cut it shorter and remounted the strap......make several loops with a thick rubber band around the thumb strap about midway of it until the rubber band holds itself in place on it's own. Then, you slide that rubber band back and forth until you get the exact fit you want for that fat or skinny thumb you have. My thumb hasn't slipped out in the middle of a fast two step since I engineered this fix on my accordions.
Get used to the rubber band and you won't have it any other way.