There are usually more ways then one to do things. Centered zipper is not an exclusion. If with the fist method the top stitching played both utilitarian and decorative roles, in this method there is a separate stitch for each function.
Right sides together sew the seam leaving the zipper opening unstitched.
Press seam allowances open.
Right side up stitch around the opening. You can go as close as 5mm off the edge. It is that easy to make a perfect frame for your zipper.
Baste one side of the zipper just inside the fold of the seam allowance. Place your running stitch between the zipper coil and the top stitch you made in the previous step. You can use a water-soluble glue or any other method you are familiar with.
Place the garment right side up. Fold aside the top layer, leaving the seam allowance with the basted zipper flat. Sew as close to the top stitch as you can. Zipper foot is a big help here. If you don’t have one place the needle in the leftmost position.
Repeat steps 4 and 5 for the other side.
That’s how the sewn zipper looks from the back. Two parallel unconnected stitches running just a tad wider than the top stitching you did in the beginning. They are also a bit longer (1-2cm) with a back stitching securing them in place.
Ready! Result is pretty much the same as with the first method, what differs is the ease with which the narrow channel can be accurately done. Centered zipper sewn this way is an alternative for invisible zipper method.