Hey Rokas,
most of the time i have buildings with combination of one or more fixed and window/door panels with different sizes..like first floor: total opening is 140cm where win/door takes 80cm and 60cm ist fixed..second floor: 200cm where double win/door takes 140cm and so on..i will create my own library with all those combinations..and my the goal is when i finish modelling the building i create shapes from openings selection, apply railclone with win/door type, and SET the door size, according to the CAD drawing.
I could also use your one spline way, inserting vertex.. in that way I need to adjust Y size and door size, right?