Dear Miriam,
I don't think what you need is a dart; I think you need to elongate the skirt. But the best way to get a perfect fit is to make a muslin. You'll need the main front and back pieces and the skirt pieces. Sew them with large machine basting stitches, so that they are easy to rip.
It's better to make alterations above the waist and above the hip, by slashing across the area that needs altering, and filling in the gap with a piece of cloth zigzagged in the open slash. Anywhere two pieces have to sew together must be square--that's why doing the alteration this way is better. If you simply lower the back as you suggested, you will end up with a pointed waistline. The dress should be perfectly smooth, with no wrinkles, but not tight. Once you're satisfied with the look, mark all the seams with a wide marker. Also mark the darts, and make them longer or shorter, as necessary. If you have wrinkles anywhere pin them out and sew them into "darts" (not really darts; this material will be gone when you're finished). Take the dress apart, and carefully remove the seam allowances, open the darts (but not the "darts") and use this to make a pattern. Trace around the pieces with a dash line, then add back the seam allowance. You now have a perfectly altered pattern to work with. You can put any length of zipper you want in the back. Just measure the opening you need for your dress, add 7/8 inch to that length.
Good luck. Please post a picture when it's done.
Teri