extend a shape with pen, adding with or without original shape?

[Egg Bramhill]

Hi RS, I wouldn't expect you to get the shape on the right hand side in you image. It's not something I ever use but in my understanding if the additional line start/end is within less than the original shapes half length it will add the shapes. However if the start/end is more than half the originals shapes length it will subtract the original shape. Problem being with an erratic shape it's not always easy to know the half length so do smaller additions at a time.