You can verify that is the 0 mark by verifying top dead center. If that center mark is longer or larger than the others, it should be the 0 mark. Did you look VERY closely for a "0" stamped on the timing tab? Sometimes they are not stamped very well.
As to how to determine 5 degree marks, I cannot explain any better than in the video. If yours is not marked in 5 degree graduations, It's probably marked in 10 degree. So just cut that amount in half.
