The best way to hem a dress is to measure from the floor with the garment on the person. I have done alterations for years and this is the best way. Mark the hem where it needs to be, then measure down the width of hem desired and cut off the dress. You may zigzag the bottom edge BEFORE cutting it off. The next step is to blind hem by hand OR hem with your sewing machine with a narrow zigzag (not short). Be sure to use a ballpoint needle or a universal one. Best of luck.