The procedure and the observations (what to look for in the light) that Dick M posts is correct. Spot on.
I help lots of people "time" their Model A engines and the most common error I find (the reason they have contacted me) is that they have confused in their head if they want the light on or off with the spark lever all the way up. So instead of having it OFF with lever at the top, and come ON with 2 or 3 clicks, they have it ON at the top, going OFF with a few clicks. That is backwards! When they do that, they are on the wrong side of the cam lobe and the timing is way off.
As a final QC check on your timing, once you think you have it correct and are reassembling everything, gently turn the rotor clockwise with your fingers to take up any slack and the trailing edge, the lower right corner of the big brass tab on the rotor should be opposite pin #1 inside the distributor cap. If it's not close to that pin, you have somehow messed up; do it again. (I do this quick check at the start, right after I get the timing pin dropped into the cam gear, before I loosen anything up. You can spot a lot of timing problems with this one check.)
