The method relies on understanding that there is one position where both the intake and the exhaust valves are at exactly the same lift. This is because there is one position where both valves are open for a short period, which is called "the overlap." The trick is realizing that the Factory marked this position by striking marks on all the timing gears that would line up for this very brief positioning of the valves at equal lift. Unfortunately, the Factory also used the "hunting tooth" method of design to prevent wear on the gears, so the gear teeth, as well as the timing marks on the gears, line up once every 76 turns, whereas the valves are at equal lift every two revolutions of the engine.
The best way to measure what is happening is to use two digital dial indicators because you can simply zero the indicators at TDC and then watch the dials to see when the numbers match each other, say at 0.120". One dial will read +.120, and the other will read -.120. Your degree wheel should be at 4 degrees before TDC, with the exhaust valve closing. This TDC is not the one where where you adjust the pushrods...that one is 180 degees different. TDC compression is on one side of the cam, and TDC overlap is 180 degrees different, between the cam lobes. The Factory selected 4 degrees before TDC between the lobes because they liked the performance a little better.
David