Yes, I have to figure out the correct dose according to weight. If I am in doubt, I have the company advise me. I always throw away those little scoops that come in supplements and use a measuring spoon once I've figured out the amount.They can live on air, as long as the minerals are reasonably balanced!
I use a ration balancer (Triple Crown, what used to be the "30", not the Gold). Then I also give him non-iodized salt, some extra vitamin E, psyllium because he's prone to fecal water, and right now he's getting extra magnesium and chromium in the form of Quiessence.
I dose according to weight - of both the pony and the supplements. I don't rely on the companies' included scoops but figure out what a serving should be for him and use a kitchen scale to determine the volume I need to feed.