The best way to determine whether your horse is the proper weight is to carefully examine the ribs, back, withers, and shoulders of your horse. If your horse is completely normal, the ribs will not be visually distinguishable, but you can just feel them. The back will be almost flat on top. The withers will be slightly rounded. The shoulder and neck muscles will blend together smoothly, and the tail head will have a light spongy layer of fat around it.
To learn more, read Is Your Horse Too Fat
and Is Your Horse Too Thin