Mexican Redknee (B. smithi) male or female???

When you're sexing via exuvia booklung angle has nothing to do with it. What you're looking for is the presence of spermatheca between the anterior pair of booklungs; the presence of this structure indicating female. They'll look like this:

http://www.arachnoboards.com/ab/gallery/showimage.php?i=5696&c=10

From the lighting of your shot, I see what could be the shadow of the spermatheca (as if the structure is sticking straight up to the camera), but i'm leaning towards male.

I've done some research and the angle DOES matter, its not 100% like the spermathecae, but it is a pretty clear indicator, just like the width between the booklungs.

A male's vent will be directly between the booklungs (top pair) usually, horizontally. A female's will be below creating a curve from bottom of booklung to vent to bottom of second booklung.

To get a better shot for the spermathecae sexing should I shoot down from the top (using this picture as a map) or up from the bottom?

If that indeed is the spermatheca's shadow, i'd try taking one slightly angled from the bottom. If you see a spermatheca its female, if not, its a male.

Of all the pictures of guaranteed female B. Smithi on here, exactly NONE of them have visible spermathecae... is this one of the species where you can tell by the flap, not by visible spermathecae???

If so.... is that a flap???