I've always wondered, why do you have to start every word with Initial Capitals?
With a small pony guessing bodyweight is likely to be inaccurate, you would do better to take it to the vet and get it weighed properly and then asking them for feed advice.
Well...that's 4.125kg in total, and 20% of that is 0.85kg in hard feed per day. But this is the roughest thing ever, just my guess! if it was me, I doubt i'd be feeding any hard feed tbh, as it's not working that hard.