The LCA is not based on nation wide prevailing salary. It's based on local avrage salary standard for each position.It differs from state to state. Of course, it may not be so detail to fit all the titles & positions.
So, you may want to find a catagory that is close to yours and try to fit in. If you have help from a lawyer, just ask the lawyer to provide that info. Normally, they will purchase those data from private agency or get them through some channel for more accurate assessment.