Matthew - a couple ways I have done it:
1) make your cut, then at the top you cut a short 'Y shape so that yu have two short cuts at 45 degrees either way from your main cutline. Then when you sew in the zipper, having first determined the right amount to fold over and the right point on the ziper tape to line up with so that the exposed zipper equals the amount of fabric that is folded under (hope that makes sense), when you get to the top you'll have a little triangle that folds up instead of off to the side, and that finishes it off pretty well.
2) Even better is to continue your cut to the top of the fabric panel, and from the top of the zipper to the edge of the fabric, you insert a narrow strip of fabric(same width as the zipper) with a hemmed bottom end that laps down over the top end of the zipper. That gives you a cleaner termination.