My mare is cow-hocked in back so it is uncomfortable for her to stand squarely, how can i do a dressage halt?

ok so in dressage they ask you to halt and the aim is to halt squarely and on the bit. My mare, however, is cow-hocked in back so its difficult and probably uncomfortable for her to stand squarely. Am i supposed to just take the marks, or can i get a "hadicap" type slip from usef/usdf? I have tried to teach her to square up, but its difficult because she becomes uncomfortable. She's a great mare that moves wonderfully, no other problems.