In the spell description of "Call Lightning" it is said that you choose the point of impact "When you cast the spell". Later it is said that you can redo the attack as an action once per round. The big question: Will the lightning strike at the same spot, if you use it again? Or will you also be able to choose a new spot?
Personally I would say you only choose the spot at the START of the spell, so further lightning strikes would be at the same spot. I think this would make most sense with the way the spell is formulated. Also the other way round would make this spell waaaaays too powerful, basically giving a 4d10 cantrip which is something other classes in the same field of expertise (especially the evocation mage) do not have.