Unfortunately, you have to read the fine print on precuts. They usually contain 2 cuts from every fabric in a particular 'line'. The up side is that all the fabrics coordinate. The down side is that there's not a standard number of fabric patterns in a fabric line, so you might come up with a jelly roll that is 20 strips or one that's over 40. Same thing with layer cakes. That (doesn't) answer the question if the amount of fabric in the 2 precuts constitute the same yardage. The issue is the shape of the cuts. It really depends on the particular pattern. Often (but not always) a pattern calling for a jelly roll will use a strip piecing method, which won't work with a layer cake.
I would suggest looking at the Moda bake shop for layer cake patterns:
http://www.modabakeshop.com/p/recipes.html