I am not sure of the veracity of this tale, but I did finally find the spot. There is a white marble marker in the street behind the Duomo (see photo), which allegedly marks the spot where the gilded copper ball (by Verrocchio) at the top of Brunelleschi's dome
hit the stones of the street after it was struck by lightning during a storm in January of 1600. I am not sure if this spot was covered over by the construction equipment that has been there forever (not likely since it is pretty far away from the church) or if has just become visible again since the changes to the area for the new pedestrian zone and some parking and taxi stand work close to it - but I do know that I have looked for it over and over and just finally stumbled upon it the other day (maybe it has just been replaced?).