The method I’m going to tell about is a part of invented in late USSR EMKO system. It was developed specifically to use with computers so it doesn’t have “use a french curve” instructions and things like that. Isn’t that nice?
The main idea is to break down all the curves into arcs with defined center and radius. Here is how I use it in Valentina.
That’s what we start with. Н01 and H03 are shoulder points, B11_B21 is the bottom line of the armhole
 The first thing we do  find points where armhole touches the bottom line and vertical lines
Line_B11_B4 = Line_B11_B21 * 0,6 (on B11_B21)
Line_B11_H11 = Line_B11_B4 (up vertically)
Line_B12_H2 = Line_B4_B21 (up vertically)
That will be start and end points of the first two arcs
 Find center points. These two arcs are always 90° so their center points may be easily found using “point from X and Y of two other points” tool
 Now draw the arcs.
Arc tool, center point H12, radius Line_В11_В4, first angle 180, second angle 270
Arc tool, center point H22, radius Line_В4_В21, first angle 270, second angle 360 or 0
 Now find center points for other two arcs. Find the intersection point of back shoulder line (A21_H01 in my drawing) with H11_H12. Connect the point you got (H13) with H01. Line_H13_H01 is radius of two intersecting circles with centers in points H11 and H01. The point of intersection is the center of the third arc. (Н131)

The next thing is finding the angles of the arc. This time 90 * x won’t work) We will use AngleLine option in formulas, so we need to draw lines with needed angles. Just connect Н131 with H11 and H01 (Make sure to click H131 first)

Finally, draw the arc
 The process is similar for the last arc. Find a point of intersection of A81_H03 and H2_H22 (point H121). Use H03_H121 as the radius of two circles (centers are H2 and H03). Connect center point with start and end points. Draw an arc.
That’s it! A little messy (I wish I could use AngleLine without actually connecting the points) but it scales perfectly with different measurements or when you need to make armhole longer, for example.