Building Block Dress Placket

Hi, I'm confused on where to draw the line for the placket. On pg 46 it says to use the center-back fold line and illustration looks like it's from the center of the skirt using the fold line as a guide. Then what is the center fold line on the pattern that runs the length of the skirt for? It's the line about a half inch away from the fold line that has the placket dot on it.