I'm not sure what you're talking about... When using the other duct, you always end up with sub 12 cm gaps. If you use a straight duct for the outside corner, the other side of the beam will start at "offset 3(?) cm" instead of starting at 0cm. So when you reach the next corner, you are either 3 cm too long, of you have a 9cm gap you can't fill as the shortest duct is 12 cm.