saddle placement question

[RhndLev@cs.com]

As long as I'm bugging people with saddle questions, I might as well ask 
another that's been bothering me.  I've been riding my mare in a Wintec 
dressage saddle.  I put it where I *think* it ought to be and it slides back 
during the course of our rides to about a hand's width behind her shoulder 
blade -- about four or five inches behind the top of her withers.  When the 
saddle is moved back this far, Special's shoulder blade comes within and inch 
or so of touching the front edge of the saddle when she's really moving.  
Moving the saddle any more than inch farther forward and her shoulder will 
contact the edge of the saddle when she is in gait.  So, is this too far back 
for the saddle to be?  It feels as if it is too far back based upon the 
western riding lessons I have taken, but I've never ridden in a dressage 
saddle before and just don't know.

Rhonda