There really isn't a fool proof measurement. I always recommend the shortest girth possible for short billets - one that will JUST do up when the horse is at it's largest/widest and a pad is used, but I supply girths if wanted when I fit saddles so people can try them for size. Girths are cinched up and snug, no measurement you can take can replicate that, many girths are handmade and can vary in length (and some manufacturers do not measure them buckle end to buckle end)...the variables are endless. 54" is a pretty long girth though, my most popular size is 46" but I do fit a lot of cobs and ponies, I seldom sell any that long, though I do have two horses on my books who need 56" girths.