Most supplement labels do have feeding directions that are based on weight - often one "dose" is for a 1000-lb horse, they don't usually break it down any further than that. So, divide the amount by 3 or 4 to get a mini dose, in general, depending on how much your horse weighs. It's best if you can weigh it initially and figure out what the correct amount looks like, a gram scale or good kitchen scale works well, has to be fairly sensitive though because mini-sized amounts of most supplements are going to be just a few grams.