How well are the ground tracks for solar eclipses known in advance of the event?

The positions of the Sun and Moon are known to better than 1 arc second accuracy. This means that on the Earth, the location of the track of totality is probably known to about (1.0/206265.0) x 2 x pi x 6400 km = 0.19 kilometers or a few hundred meters at the Earth's equator.