I'm not feeding it currently, but when I did, I fed up to 1/4 cup for my B-size minis (half that for my little stallion), depending on what they needed and what I was hoping to get out of feeding it.
I also fed 1/4cup (or when I got lazy - a handful) of Boss back when I fed it. I loved the way it made the horses coats shine and darkened the colours (maybe darkened isn't the right word ... enriched ?) but with minis being so hard to keep excess weight off of (most of mine at least) I found it too calorie dense for them and discontinued it because I was seeing cresty necks and worried about obesity related problems. I now offer about the same amount of flax instead, get the same good results without the weight gain.