This is from Dover’s guide to fitting a show coat:
“Viewed from behind, the hem of the hunt coat should in general fall at a point where two to three fingers’ width of your bottom is visible. However, some people prefer the hem to fall at a slightly lower point. If the hem appears to be correct, but there is a horizontal bulge of fabric running from hip to hip across your back, then the jacket is probably too long. Try a jacket in a short length or one from another manufacturer. Conversely, if the jacket is too short (you can see most of your bottom) try a jacket with a long or tall length.”
And this is from Maryland Saddlery:
“For a traditional fit, the length of the jacket should just pass the bottom of the rider’s bottom. When seated in a saddle there should be 1 ¼” of extra fabric lying on the saddle. For a modern fit on a tech coat, the length is determined by the cut of the coat. Some may be as high as totally above the rider’s bottom."
I prefer the look of the “modern fit”. 