Along with the shoulder , humerus angle, and length I ask that you also look closely at how these three horse neck sets , length of neck and attachment of head to neck.
 

Front End #1

Front End #1 In this picture the horses right cannon is not set under the horse, but back so we will work with what we have here. This horse has the steepest shoulder of the three horses pictured. Being about 56 degrees. This horse also has the the most horizontal (but still moderate) and shortest humerus of the three. The elbow on this horse is also set close to being under the crest of wither. Putting these combinations together makes this horses movement in the front to have moderate  reach  and moderate upward lift of the front legs. This horse has a between  medium and high set of the neck in the shoulder and the neck looks to be on the short side. The throat latch on this horse looks to be just a bit thick. Also looking at this horses back dropping off from the wither, looks to be more to a ventroflexed back. This structure would lend me to believe that this horse would go towards the stepped rack. This horse has good muscling of the shoulder and a deep heart girth making for a horse that has lots of stamina if kept in it's best gait for the structure.

 

Front End #2

Front End #2 This horse has about a 50 degree shoulder angle and the humerus length is more than 50% of the shoulder length. The humerus is also steeper than horse #1 but still not considered a steep humerus. The wither is set well behind the elbow of this horse. The back on this horse looks to be towards level. This horse should have good forward reach of it's front legs. The neck is also set in the shoulder at a medium height. This horse looks to have a nice balanced length of neck and very clean lines of the neck . The head is set on well , being clean in the throat latch , not thick or thin. This horse looks to me to be able to do a very nice ground covering gait. A running walk if the gait genes and hind quarter structure is there to support this. The  length of neck and the way the neck is set on would help this horse to have a very nice loose head shake.

 

Front End #3

Front End #3 This horses front legs are set out to far forward to do a good evaluation but we can see the shoulder angle well. It is like horse #2 at about 50 degrees. The humerus length is just a bit shorter than horse #2 but longer than horse #1 and at a steeper angle than both but still not considered overly steep. This horses humerus is also more than 50% of the shoulder length. Due to the angle of shoulder  and length of humerus we should have some reach in the front legs but the steeper angle of the humerus should give a bit more lift of the front legs. This horse could  do 2 different gaits  fairly well. The front legs setting forward make it hard to tell how the back is set in this horse but I think if they were brought under the horse with the cannons being vertical the back would be more to level. The biggest factor in this horse in relation to differences of gaits executed is how the neck is set on and the neck length. The neck in set high into the shoulder and is short and thick in the throat latch. If this horse is asked to carry it's self ventroflexed (hollow) it could go toward a ventro gait such as the  rack.  If the horse is held to a more level back it could do a running walk with a bit more lift in the front and have more of a head nod than head shake. This horse has nice depth to the heart girth and lots of shoulder muscle making for stamina and front end strength.

It is always interesting to see all the different combinations of structure you will see on one animal. This is what makes it fun when one has the opportunity to see everything put together front and back and see what transpires in relation to achieving gait. Looking at many horses and as many different gaited breeds as possible will help you
to get a good Idea as to why a specific gait is possible or not.
 

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