[olddog]

The timing light flashes when the plug wire it is connected to fires. Your eyes see the crank shaft mark, when it flashes. Where the engine was, when the distributor was inserted, has absolutely nothing to do with the accuracy of the timing light.

If forger the number of teeth on the distributor gear, but you're just trying to get the right tooth lined up. All you can do is move the distributor one tooth +/- at a time. You cannot align it any finer than one tooth. That is why you rotate the distributor with a light. Frankly rotating the engine 10 deg doesn't amount to a hill a beans.

What is most important is that number 1 is on the compression stroke when you put the distributor in, because if it is not you will be 180 degrees out of time.

If this is a fuel injected engine, which position in the distributor cap you choose to put #1 cylinder in matters! On a fuel injected engine, you have to set the distributor up correctly because it also controls when the injectors fire.