This is a rather interesting question, one which I struggled with for years before I understood Matrixing. After all, different arts say the horse stance should be according to the standards of their particular art, so which is right?
Which is right depends on what you are doing with the horse stance.
If you are fighting, you must match the horse stance (Kiba Dachi in Japanese) to the correct height depending on what you are responding to or initiating.
If you are training, you must seek balance between too low and too high. I know this sounds loose, but remember that you are trying to find the balance between attaching yourself to the earth, and being able to move on the earth. As the situation changes, you must change your horse stance.
One of the things I did, way back when, was take a photograph of myself in horse, and draw lines over it so I could understand the angles of the stance. Another thing I did was the horse stance form endlessly. This is also known as the Tekki forms.
One thing I did was eliminate the outward splaying of the toes in such systems as Goju, for this destroys the connection to the earth. And I did other things as I progressed through my analysis of the stance. Prime among them was the set up for change from and to other stances from the horse stance.
At any rate, I can only give you these guidelines, and let you solve the problem for yourself.
Drop by Monster Martial Arts, I’ve got a lot more data on this stuff, and there is a free book offered at the top left of the home page.