Aligning Separating Zippers Correctly At The Top/Bottom Garment Opening
Sewing rule: One of the most important aspects to sewing a good quality separating zipper, is how evenly it is aligned along the garment opening. When the zipper is fully closed, the distance at the top and bottom should be equally positioned along both garment edges.
The seam allowance distance left at the zipper's top portion should not only be equal on both edges, it should also be sufficient for the neck finishing technique used for the garment at hand. It often safer to leave a longer distance than the neckline's designated seam allowance as this will allow more comfort and precision during the neck finishing process.
Similar to the top, neckline edge, the bottom of the separating zipper should be positioned at at equal distance along both edges and consider the hem finishing technique to be applied. For example, in the example pictured above, the hem will be finished with a facing, thus only 1/2" hem allowance is left to accommodate the facing-garment connection seam. Regardless of the hand finishing technique and how wide the hem allowance is, the zipper's bottom stop should be positioned right above the hemline and not extend into the hem allowance as this will disrupt the process of clean-finishing the hem.