This shows the essential parts of the stock factory ignition setup. The computer is between the distributor and the ignition module, allowing it to play tricks with the spark timing while it controls the fuel mixture too.

• The engine should be completely warmed up.
• Disconnect and plug the vacuum hose at the distributor.
• Unplug the three-wire connector at the 10" and 4" Vacuum Switch Assembly.
• Loosen the hold-down bolt and rotate the distributor to set the timing.
  

On my engine, the first large mark on the scale is zero, the next large mark is 4, the next is 8, etc. Don't forget to reconnect the vacuum hose at the distributor and the connector at the vacuum switch assembly.

Your buddy borrowed your timing light and never returned it? Here's a trick I learned from my brother. The timing can be set to surprising accuracy with a vacuum gauge. Hook the gauge to manifold vacuum and run the engine at curb idle. Rotate the distributor clockwise to retard the timing. Now slowly rotate it counter-clockwise to the point where you just reach the maximum vacuum reading. Back off (clockwise) until the vacuum drops by 1". Lock the distributor down. Timing can be set to within 1? using this method.