Most all modern cam makers use a .002 and .050 measurement for their duration figures, but Ford used a much earlier opening figure back in the day (instead of the .050). I can't remember it right off hand, but I believe it was either .006 or .010. But those measurements I suggested will allow you to compare figures to both Ford and newer grinds.
Yes, opening point is the exact point at which lifter rise occurs.
You don't want to use the division method to determine the center of the lobe. Reason being that the front side of a lobe may differ from the back side. The best way is to find the point of maximum lift, noting the spot on your degree wheel, finding it on the other lobe, then determining the difference in degrees.
Edit to add that it can be a bit tricky to find the EXACT point of maximum lift when you have pistons and rings in the engine. It's not easy sometimes to just nudge the crank 1 or 2 degrees with the drag of the rings. That's why this stuff is probably best done with no rotating assembly other than the crank. But with some patience, it can be done pretty accurately.