a bit off topic, but you don't really need helical interpolation to mill an npt thread. just cut the thread into 4 90 degree arcs and adjust the radius for the taper.
like this
G01Z0F30.
G01G41X-.301D28F5.
G03X0Y-.3014R.3014Z.0139F10.
G03X.3017Y0R.3017Z.0278
G03X0Y.302R.302Z.0416
G03X-.3024Y0R.3024Z.0555
G01G40X0F15.