Grid and the grid check

The grid is the size of the squares the board is cut into, from 1 mm down to 0.25 mm, and the grid check under the map says how much a coarser grid would change the answer.

Start on Normal, and choose a finer grid when the grid check reads more than a degree or two.

A finer grid takes longer and cannot fix wrong watts, so check the inputs before you reach for Finest.