I wouldn't order a corset by your back size. Some corsets are done by back size, like the dream girl corsets... but these are real corsets as they won't pull in your waist as they are made with plastic boning rather than steel. If you order a real corset, I tend to go a few inches below my waist size. As I have a size 24 inch wait this means I will order a size 22" or 20" corset, depending on the number of steel bones and how much I want to take my waist in. Corsets tend to allow room for the bust due to how the boning is shaped, and unless you have a very big sized bust compared to waist the corset will be able to squeeze your boobs in and form a nice shape.
I have a few 24 inch waist corsets which close completely on me due to my waist being 24 inches, they also close completely around the top of my bust... this doesn't mean I have a 24 inch bust, it's just how corsets are shaped to only reduce the waist and allow room for the breasts and hips.
LH doesn't actually sell any real corsets, they only sell bustiers and basques, which are based on your clothing size or actual wait and bust size. I think Hella was taking measurements of all the ones on LH and you should be able to find sizes for the bust and waist in the descriptions.