Bar spread - seeing what you are feeling for
We are no longer building saddle trees, but we have two videos about how Western saddles fit horses available on our westernsaddlefit.com website.
We looked at what you are feeling for when checking out bar angles. But what are you feeling for with bar spread difference? (Bar spread is the distance between the bars - how far they are set apart. This is determined by the hand hole measurement as the gullet width measurement is useless in practice .) From the side, this tree looks ok just sitting on the horse. But let’s take a look from the front. (Remember, you can’t see this in pictures or even in person on a finished saddle. You have to feel for it!! But these pictures are meant to help you know what you are feeling for.)
From the front, you can see that the bar of the tree is sitting quite high on the horse. Now remember that you can’t tell this by the amount of clearance, because the clearance is set by the height of the gullet and hand hole built into the fork. But when you look at the picture you can see that the angle matches the horse very well. (It could be a tiche wider, but this is more than adequate an angle for this horse.) There is even contact from the bottom of the bar up until it "runs out of horse" to sit on because it is sitting so high on the horse. The top of the bar is off the horse with no contact there, just because the spread between the bars is too narrow. 
So will adding more padding help? No, because it just lifts the tree even higher off the horse, meaning less of the bar is on the horse. Here we have mimicked having skirts, shearling and padding under the tree by adding padding. You can see how much the tree has been raised from the previous picture by the increase in clearance, so you can imagine how high it is relative to the horse now. There will be minimal bar on the horse at this point. Adding padding doesn’t help a tree with a narrow spread fit better. It makes it worse. Too narrow is too narrow and you can’t do anything about it.
In contrast, look at this tree on the horse. Again, from the front the bar angle matches the horse.
But look at the side. The tree is “falling down in front”. It is sitting much lower at the front than it should be. The clearance is low, but remember, we can’t tell proper spread by clearance, because that is set by the gullet height, but it can give you an indication that you need to be checking bar spread (and angle). So looking at where the bar is relative to the horse’s back, it is tipping forward. It isn’t matching the slope of the back.
So will padding help this situation? Actually, in this case yes. Extra padding will fill in the extra space between the bars and, because it lifts the front more than the back, it will help this tree/saddle fit much better. (The same wouldn’t be true if it was too wide a bar angle because padding doesn’t change angles.)
So again, here are pictures that help you see what you are feeling for when you check a saddle on a horse. Check for matching "angles", and for where the bar sits on the horse's back. Is it too high, falling down in front, or just right? (Note, you do want it a little low without padding so with padding it is matching the slope of the horse's back.)
