Getting the dimensions right and checking the squareness of the opening are the fundamental first steps of any installation. The overall dimensions of the rough opening should be checked by taking both the vertical and horizontal measurements at the top, midpoint and bottom of the opening.
Then check the squareness of the opening by measuring diagonally in both directions. The measurements should not differ be more than 3mm (1/8”) at most.
These dimensions should then be checked against the dimensions of the window itself. There should be a gap measuring about 13.0mm (1/2”) wide around the perimeter of the window. This gap will allow for movement between the window and rough opening as well as room to level the window, place proper shims, and properly insulate around the window frame.