http://www.lowes.com/cd_Install+an+Exterior+Door_554148183_2. Measure the height, width, and depth of the door opening. he new door should be ½” less than the height of the rough opening and ¾” less in width.
Would you not require a door to be less than the door opening in height and width by perhaps only 1 or 2mm?
Less by 1/2" in height and 3/4" to me would seem like a very large gap between the door and the frame.