Lots of math. Most supplements are based on 1000-1100# full size horses. For small minis, 34" and under, I usually give 1/4 full-size supplement dose; for large minis, 34-38", I usually give 1/3 full-size dose; if the mini is really small like 30" and under, I give 1/5 dose. If there are instructions per 100# BW, I follow those directions.
I've kept scoops from various supplements, and even bought a few really small scoops, so have scoops in many sizes available, so I can usually dose down most supplements decently, even without a small gram scale (I have a postal scale, so it does ounces and pounds, not small increments). One tablespoon equals 3 teaspoons. 1 teaspoon is 5ml (5cc), 1 tablespoon is 15ml (15cc). Seems many of the scoops are in cc or ml measurements (some of the larger ones are ounces or cups). About 454 grams in a pound.
We can start here.