Measuring Your Horse’s Hoof Structure and Growth:
Frog and Heel Bulb Health, Shape and Proportion
Measuring width of heel bulbs at back of frog.
The frog and heel bulbs play an important roll in hoof soundness. One important function is that they help absorb concussion when the hoof hits the ground. The picture below shows nice wide, healthy Heel Bulbs (reference A).

Slighty indented central sulcus. Beautiful! (reference B)
Measuring length of frog. (reference C)

A wide, tough, leathery surface with a slight indented central sulcus in the middle is needed. This area of the hoof needs pressure and contact with the ground for strengthening, callusing and blood circulation. If this does not occur, this area of the hoof will atrophy, deteriorate and harbor bacteria. As a result your horse will not make a desired heel first landing. The health of this area attributes to the soundness of the internal structures.
Measuring frog/sole proportion.
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Researcher Dr. Robert Bowker has noted the presence of pressure receptors in the frog that help in sensing different types of terrain and surfaces. He also recommends working towards a one-third to two-third frog/sole proportion. The length of the solar surface should be at least two-thirds frog and measuring from the apex of the frog to the end of toe should be one-third.
Measuring depth of infected central sulcus.

This is an example (reference D) of an unhealthy frog with a deep infected central sulcus.

This will improve with:
1. Soaking to kill the bacteria/fungus.
2. Proper trimming to allow de contraction
3. Allowing frog pressure for better circulation to grow healthier tissue.
Keep a measurement log to track progress!