One other thing that you may find helpful is a 'hoof meter'. My barefoot teacher taught us by tracing 35, 45 and 50 degree angles on a piece of clear plexiglass, you can measure the proper angles of the hoof before and after the trim. The 35 degree is to be measured at the cornet band to check the proper bone alignment inside the hoof. (you can't change this but it helps to understand the hoof structure). The 45 degree for proper fronts angle and the 50 for proper hinds angle. Hope this is useful.
The above angles(which are different also depending on whether the foot is weightbearing or not) are *averages* of a 'normal' range. Also, when talking about wild horse studies - the main source of 'normal' angles - I believe studies have only been done on a relatively small scale, of horses who live in similar(rocky, dry) environments. For eg. On speaking to people who have a lot to do with brumbies who live in sandy country, flatter, shallower angles are the norm for healthy feet. So while understanding what 'normal' angles are is helpful, it needs to be used as a guide, nothing more.
As with most of the specifics of trimming, there are plenty of principles, but few if any hard & fast rules. So many exceptions. That's why I feel it's so important to find and make the most of an experienced mentor. If it were about 'proper' angles & lengths etc, we could all learn well enough from a book. Pity it's not that easy, huh??