Hi , I'm putting a lintel in and need to cut out the aperture for door . I wondered if there was a recommended standard height between the floor to the under surface of a lintel for a standard door(1981mm high)? Do you simply add the height of the door(1981mm)and the thickness of the door frame at the top?(27mm) , and use this as you aperture height ?, and then trim the door to suite flooring thicknesses and gaps,etc..?