I made this prick punching aid for a friend many years ago. Once made, I decided I needed one too. The body of the tool is a through-drilled cylinder with one quarter of its body accurately milled away.
Insert the punch, made from hardened drill rod, into the hole, align the edges of the cutaway portion with your scribe lines and voila (not viola - that's a musical instrument), the punch mark is on the intersection of the lines.
But wait, there's more! (He shouts, madly waving his Ginsu steak knife.) Ever had the problem of having to align to the edge of a piece of stock angled in the milling machine vise? Drop the body of the punch over the edge and use the edge finder to locate on its circumference. Since the diameter of the punch is known (or can be easily measured), it's a simple matter to locate the spindle axis directly over the edge.
The one shown is massive enough to not move under the friction of the edge finder but, if you have problems, apply a dab of oil or grease to hold it in place with the capillary action. (I have a miniature version that requires this action to remain in place.)
