Timing mark question??

Went to set timing today on the 89 Jetta, 8v 1.8 automatic. There are two marks on the flywheel about 1/2 inch apart: a straight line, and a 0. Adjusted to either one and she runs fine. Question is: which one is the correct one to time it to????

With engine warm & at idle align to the line. The 0 is top dead center.