1990 Lincoln Town Car setting timming

I need to know how to properly set timming. I have a timming light and know how to hook it up the problem that I am having is understanding what mark to line up with what part of the tab or scale. There is a flat end on the scale that has the word TIME from there the scale tapers down to a notch there are no other markings on the scale just a hole where I would gess that a tech would hook up a probe or sensor. I would appreciate any help on this topic