I recently bought a timing light off ebay (without instructions).
It has the dial advance on the back and I understand how this works.

Here's my question. The dial goes from 0 - 60 degrees, but it goes
slightly past 60 and slightly under zero. How can I calibtate the dial
to ensure that zero is actually zero on the dial. I don't have access
to a conventional timing light to check the zero spot.

Thanks.