You don’t have to do that. What you’re actually doing is putting a kink in the curve. For the control points for the curve end points use an angle formla to make the angle perpendicular to the shoulder seam. In your case you would add or subtract 90 from the angle of the shoulder seam… I can’t see all the points and don’t know which way the line is going to be specific… but it would be something like Line_A?_A12 + 90.
Also just a note… not all armhole / shoulder seams end at a right angle (or for that matter any seam / seam or seam / hem)) … especially with period jackets with drop shoulders. For ex:
What’s important is that the angles on either side of the seam add up to 180 degs NOT that they both have be 90 degs.