Not a trick so much as a hand work approach instead of mechanized
one. If you want to keep the locations precise and symmetrical put
down the flex shaft and pick up a file.
Try this: Layout positions, mark centers with scribe, incise scribe
marks with an upwards push stroke of saw blade to make a notch. Saw
the notch mark deep enough to use as guide for a file. Use a small
round file (needle or escapement, depending on the size of wire and
notches) or either a joint edge or crochet file clamped in a saw
frame to keep the grooves perfectly centered and aligned.
In this type of work requiring precision, the expediency of a couple
seconds with the flexshaft is not an adequate replacement for a
minute or two of proper hand work with a file. Once you have the
center locations marked there is no need for them to end up off
Michael David Sturlin