I agree with this method with one enhancement. At TOD the up and
vertical movement of the piston is at it's smallest. In other words
it's hard to tell the difference between say TDC and 5 degrees off
TOD. A more accurate way is to look at the piston travel at 90 degrees
of TOD where the piston travel is it's greatest. Using the same
technique with the screwdriver or pencil. Mark the pencil against the
spark plug hole anywhere around 90 degrees of TOP and mark the
flywheel. The actual distance from TDC is not critical. Now turn the
engine the other side of TDC until the mark on the pencil is exactly
in the same spot against the spark plug hole. This needs to be
accurate. Sometime I shine a light across and let the shadow line be
my guide. Mark this spot on the flywheel. TDC is exactly half way
between the two marks on the flywheel. Check your work by going to TDC
and looking at piston travel.