To judge a downhill horse....it's not necessarily what it looks like across the top,.....as in the very top of the croup measured to the top of the wither, it's more than that.
"In order to determine whether your horse actually is balanced downhill, you have to remember that the actual location of the lumbosacral joint is about 4 inches below the point of the croup, so that the appearance may be deceiving.
First you have to find the widest point at the base of your horse's neck by placing your palms against each side of the neck and sliding them downward until you find the widest point, and mark the surface at that point. This point should overlie the junction of the 5th and 6th cervical vertebrae deep inside of the neck.
On the same side of the body, walk back and locate the point of the croup, then measure downward from that level to 4 inches below it, and mark that point on the horse. This marking should correspond to the level of the lumbosacral joint. Stand the horse square on a level surface, and connect the two points. If the line connecting the 2 points is within 4 inches of level, the horse is balanced within normal range. If the line angles sharply down toward the front, the horse is weighted toward the fore with downhill conformation."