You will want to find the position of your shadow mark when the sun is the highest in the sky.
This does not occur at 12:00 noon, even if you are in the exact middle of a time zone which is perfectly sized and spaced for this to be true.
A better method is to plot the height of your shadow mark for several minutes before and after 12:00 noon (standard time), note the time when the shadow mark reaches its peak, and then use that time.
If you have a small point shadow you can make a vertical line on your chart through the shadow of the small point when this peak time occurs. Then you will not need a clock but can make your marks on successive days just when the small point shadow crosses the vertical line. (The very small error caused by the slight change of mid-day throughout the year can be ignored.)
Of course, you will not want to be fooled by daylight savings time. :=)
Cartoon Courtesy of Grimmy, Inc. Copyrighted 10/30/2006