I also second SmartPak's supplement line. They are a fantastic company to work with and have very knowledgeable staff. Their products are some of the best out there in my humble opinion. However, I tend to take manufacturer's recommendations for feed with a large grain of salt with all my animals. I learned that lesson when I started reading said recommendations for my dogs and found out that the manufacturer wanted me to give my pair of 75lb dogs like 4 cups of food a day!!!!! They get 2 cups a day and occasional treats and the vet says that's perfect.
That being said, if I have any horses on supplements that are more for maintenance and not a specific, chronic condition, I usually halve what the manufacturer recommends. Chronic conditions and stuff that comes straight from the vet, I do exactly what they tell me.
Hopefully that helps some.