Oh, those little triangles! Glad you asked about them.
The way I cut them is to hold the handpiece like a pencil instead of a knife. Center the point ,where you want the cut to begin, between the border lines.
While looking down the scope at the graver tip, raise the back of the handpiece until the face of the graver is perpendicular to the work. You won't be able to see the face or the point, just the top of the graver.
Apply the power and plunge to the line and STOP!.
Be mindful of the real huge and sharp triangular burr you just raised---it will cut you.
Make a little back cut to remove the burr and clean up the line. Some metal will be displaced, but light sanding will lower it flush.
Repeat as needed.
Love the work, Brian. The topstrap treatment is very much like the treatment I did on a pair of Ruger Vaqueros, but mine were flare cut. I guess great minds think alike. At least some of the time!!