Ok, here I have set up the basic scenario (a few things not included):
Now I’m going to use the rotate tool:
The help in the bottom left of the screen says to select the objects that you wish to rotate, while holding down the Ctrl key and then press Enter.
In this case, I am only selecting b4 & P1.
I then move the selection to more-or-less the place that I want it to be & click the mouse button (see help in bottom left):
And this box pops up:
Since the angle really isn’t correct, I’ll need to change that by putting in the formula, so I click on the f(x) button and select the line angles that I need:
And this is my result:
As you can see, P1 has moved onto the line A1 to P2 and now has a1 added to the label & the P1 has also moved by the same distance and also has a1 added to the label.
Now all I need to do is to draw in the new lines:
Just for neatness, I’ve made the arc shorter & more centered over the shoulder line & changed some of the lines to dots or dashes. The actual shoulder line and dart legs, I’ve done a solid line in black so that I can see which points to select when I do the details for the pattern pieces later.
I hope this makes it a bit clearer for you