Actually faggotron was correct in his answer.
Heres a diagram explaining:
<img src="http://img507.imageshack.us/img507/6259/itworksca1.png">
So the maths would be something like:
Set value of myangle to abs( angle(Bx,By,Ax,Ay)-angle(Bx,By,Cx,Cy) )
If myangle > 180
----- myangle = 360 - angle
Or if you want to do it in 1 event with 2 actions:
myangle = abs( angle(Bx,By,Ax,Ay)-angle(Bx,By,Cx,Cy) )
myangle = myangle > 180 ? 360 - angle, angle