Thread milling ?

[Leadhead]

I simply drill a hole in some MDF about the size of the cutter dia. Harden it with CA glue and finish bore it with the tool. Then measure the hole carefully. This gives me my offset.

[Leadhead]

Should have said to be careful with various tool tip radii. These will of course vary the cutting dia and thus the depth of the form when trying to match an existing thread. Imagine a huge tip radius verses a tiny one. The V form would cut in different places. Start small and creep up on it.