The underside of the pommel should clear the withers (the cantle is the back of the saddle).
Put the saddle on her with no pad. Make sure the front conchos are sitting just behind her shoulder blades. When you look at the front, the front edge of the skirts should be following the line of her shoulder (same angle). You should have 2-4 fingers of clearance between her withers and underside of the pommel.
Next, run your hand under the saddle from very front to very back. The saddle should be setting evenly against the back of your hand and you should feel the same light pressure all the way back, no tight spots or air pockets.
Finally, step back and look. The seat should be level, the back of the saddle should NOT pop up off her back, and the edges of the back skirts should be no farther than mid-flank. If they cover the whole flank, they could interfere with her hip movement.