Hi, did you find my sollution I posted here?
I did this for a dart on a curved waist seam, but you could do this for an armhole too. This is actualy like the paper method.
in short:
- first you copy the needed points - I do this to keep the original clean
- fold the pieces together.
- draw the curve
- split the curve a the dart
- recreate the curved pieces and dartlegs in the original
- clean up (hide) the copied points
Good luck