First of all, you need to know exactly what your feeding. Minerals and Vitamins need to complement a feeding program. So, if you are feeding manufactured feeds, you need to know exactly what is in them already. Many are fortified with Vitamin PreMixes and adding more Vitamins only messes a horse up.
Then, you need to know the mineral content of your forage. For example, if you are feeding alfalfa, you are getting plenty of Calcium.
On the other hand, if you are feeding grass, grass hay and/or any grain products, you are feeding a lot of Phosphorus and are probably not getting enough Calcium.
You cannot get any mineral OUT of your feed, so you have to supplement what is needed by adding other minerals to your feed. You cannot get the excess Phosphorus (P) out of grass or grain, so you need to add enough Calcium (Ca) so that the horse is getting twice as much Ca as P.