PS - B and C are both far too low to ensure gut health for ANY horse and A is an absolute minimum which would only be applicaple to horses in very hard work who are receiving large amounts of high energy feed in addition to forage.
OP half is the minimum eg for a racehorse, two thirds if the horse is doing some serious work, one third is inappropriate for anything. As someone else said, as much as possible is best, only feeding hard feed if the horse loses weight or lacks energy because ad-lib forage isn't providing enough calories.