Vertical Sundial in Action - AM Only (Shaded After)

11:18am local time on phone; 14-min delay based on longitude and Greenwich meridian data, so sundial should show 11:04. Gnomon to dial plate ratio was too large, so gnomon was cut but not enough! How could you calculate shadow lengths to preclude this?

11:18am local time on phone; 14-min delay based on longitude and Greenwich meridian data, so sundial should show 11:04.  Gnomon to dial plate ratio was too large, so gnomon was cut but not enough!  How could you calculate shadow lengths to preclude this?