First mill project - slitting tool advice

[whitmore]

Sorry to be dense but I'm not following. Here you can see the part installed in a 1/2in collet. The flats are not visible.

Since the faced end IS visible, you can scribe a line there, parallel to the flats. Me, I'd just accept an inaccurate
angle. I assume this part is usually hidden?

 

[r-mm]

yep, I took this good enough approach and it seems like it was, in fact, good enough.