I haven't done a lot of taper thread milling for a while, and when I do, I use a graphical method. AND, I use a thread mill
I drew a single turn spiral helix (back in OneCNC XP this was), added a radial lead in/lead out to each end of the path to create a smooth center start routine. I cut from the bottom up.
I kept a template toolpath which I imported into the drawing. The template was the simple wireframe geometry described above, drawn at the proper rate of taper increase of 3/4" per foot, and with a 1" diameter at the bottom. By means of independant scaling of all 3 axis, I could modify the template to match the desired pitch and diameter of any size thread + thread mill combo. I placed this properly scaled bit of geometry at the bottom of every hole to be milled on the model.
The spiralling path as drawn consists of 3 or 4 blended tangent arcs of gradually increasing radius. But this is interpolated into linear segments when the toolpath is posted. The arc fitting tolerance can be set in the post to whatever is desired.
Use 'cut chain variable Z' on each instance of the path to be machined. It is not really all that much code per hole.
I suppose if I were creative, I could figure a way to do this with a custom drill cycle (since the routine is center start) and a G52 at the front of a subroutine.
EDIT: I went and looked at XR3 just to be sure, and it will draw the exact path required without importing a template, including the lead in and lead out. The only thing that is uncertain, is what the diameter of the helix should be drawn at, due to the unknown of the thread mill tip diameter.
So the method I would use is to mill the first one in a piece of scrap to obtain some sort of reference by gauging the engagement of a plug in the trial cut. Then, redraw the spiral helix a second time with adjusted (corrected) radii, and it should be good to go henceforth.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)