No, you do not need to add anything for the edge overlap yourself. The roof will overlap the structure by 5 cm on each side. This border width of 5 cm is calculated automatically. The measurements to be entered are the internal dimensions of the opening.
It is understood that this 5 cm border is added to facilitate installation, making the element easier to stretch and secure by folding over the edges of the extreme rafters, and that any excess can be trimmed with a regular utility knife after installation. However, the specific details that this edge welds two films together, or the explicit process of folding over rafters and trimming, are not detailed in the provided sources and should be independently verified.