How much Fibregenix should I feed my horse?

Whatever the bodyweight is, round up to the nearest 50g. So if you have a horse that weighs say 420kg, round it up so that he is being fed 450g per day.  This will ensure that he is getting the right amount of everything.  This is particularly important if the weight has been established using a weigh tape, as these can often underestimate bodyweight by several kg.