Handfiling a teardrop shaped Mandril

I’m in the process of making an Achtpanzer chain and the instructions call for a teardrop-shaped mandril. While I have quite a collection of mandrels, none of them are teardrop-shaped. I do however have a 10 mm bolt; a hack saw and some hand files.
The technique used for filing something round, oval, or in this case teardrop-shaped is of the divide and conquer variety. First, you mark one facet just touching the shape hiding inside, and then file it with a rough file. The idea is that a flat facet is much easier to evaluate by eye than something round.
Then add a facet to the corner of the first fact and so on and so on.
Evaluate the flatness of your facet by comparing it to something, like a straight edge, a ruler, or the side of your file.
And after a couple of hours, the shape is just about right and you can shift to a finer file and finally to emery cloth.
The book in the video is "Handgefertigte Kette" by Barbara Reichmann. It's in German and as far as I can find out, it is out of print. The only place I have found it is here:
https://www.goldschmiedebedarf.de/pro...