What is the ratio of curing salt versus table salt in a recipe?
I agree with Athanasius, it's best to follow a reputable recipe and not mess with substitutions when curing meats. You're playing with fire. The very fact that you would ask in that way tells me you don't have the background to even be thinking about it, since there is more than one type of curing salt.

From Wiki  


  Curing salts are used in food preservation to prevent or slow spoilage by bacteria or fungus. Generally they are used for pickling meats as part of the process to make sausage or cured meat. Curing salts are generally a mixture of table salt and sodium nitrate. Common types of curing salts are Prague powder #1, which is 6% sodium nitrite and 94% table salt, and Prague powder #2 which also includes sodium nitrate


It doesn't get much more reputable than Michael Ruhlman, and he's written a Bible on the subject of Charcuterie: Charcuterie: The Craft of Salting, Smoking, and Curing. Wow, it's even available for the Kindle, I might have to get that.

On-line, he goes specifically into soppressata.