Perhaps the most important thing to remember about wither tracings is that they are not indicative a "sure fit". Many other factors regarding a tree will determine if the fit is right. What we can deduce from a tracing is if the fit won't work. 
Below I have examples of four trees. The first tree appears to be a decent fit, but sadly, to our dismay, physical trial proved the back of the bars did not suit the horse. The second and third figures are extreme fits for the same horse - one too narrow and one too wide. We did not bother trying these trees on the horse because it is apparent that neither would be suitable.

Below, each figure explains what aspects determine the details of fit. 
1. Figure A (Good fit): the saddle bars and the horse’s body could run parallel with one another indefinitely. They fit seamlessly together with no digging. Imagine weight is pressed on the bars. The surface of the bars distributes weight evenly across the animal's back. Also note sufficient clearance for the withers. A blanket and 1/2" pad with a wither cutout will work nicely.
2. Figure B (Poor fit): the bars are too narrow for this horse, distinguishable by the bars not running parallel with the shape of the horse. The ends of the bars will dig into the horse with added weight and the saddle will become tippy (unbalanced) and risk listing to the side. This cannot be corrected with blankets, shimming, padding, etc. and will risk injury, particularly to the horse.
3. Figure C (Poor fit): the bars are too wide for this horse, distinguishable by the flair out at the ends of the bars and how low the tree is sitting on the withers. This probably cannot be padded up, due to the risk of the channel still coming in contact with the withers. There should be decent clearance for the withers, without risking contact. 

Figure C & D demonstrate a tree sent for fit before being built. The tree sat seamlessly, but fell ever so slightly low on the withers. To ensure no contact, we added a 1” pad to represent 1/2” of skirting and a 1/2” saddle pad. With the addition of the pad, the bars still fell parallel with the shape of the horse and allowed sufficient clearance (even with added weight) above the withers.
Please remember to pull your blanket and/or pad up into the gullet so as to create an A shape and avoid any unnecessary pressure on the withers.