Like Pete said, (After making sure cyl no. 1 is on compression stroke.) drop dis in at TDC with rotor at no. 1 wire on cap. Then you can get very close to knowing when the spark actually happens by what we used to call spark timing. With hold down just snug, set everything up, harmonic at TDC plus desired advance and rotor to no. 1. With ignition ON, twist distributor slowly until the spark is heard inside cap. Twist back and forth a couple times slowly to be sure you hear the spark right where you want it. A quick way to get you right in the ballpark.