There is only ONE good method for finding top dead center, and all degree measurments when working with engines are measured from that. It is called the "positive stop method", and it deals with the dwell period at TDC that you speak of, and it is used in everything from race engines worth many thousands of dollars, all the way down to our little engines.

You insert something into the spark plug hole that will not move in the slightest, but will stop the piston just before it reaches top dead center. You then install a degree wheel, and "eyeball" top dead center on the wheel/pointer. Then turn the engine over gently until it stops, and note the degree setting. Then turn the engine over gently in the other direction until it stops and note the degree setting. What you are looking for is a degree wheel position that affords the exact same number when the engine stops in either direction. Adjust the degee wheel until you have this condition. When you achieve this, the degree wheel will be indicating an actual top dead center when the degree wheel reads 0 degrees, TDC, (once you take your "positive stop" out). All measurements, whether they are for port timing, cam timing, ignition timing, whatever, are referenced from this point.

Here's a pic of the "postiive stop" that I made for my engines. It uses a hollow tube in the center so that you don't have to fight compression when turning the engine over.

AV8TOR