All we’re doing with the seed points is defining a broad corridor, several kilometres wide, in which the scale factor distortion will be close to zero. It doesn’t really matter if this corridor moves a few metres one way or another – it will still encompass your project.

Similarly in height, we’re defining the level at which the scale factor is unity: above or below this by a few tens of metres the distortion will be very low.

Once the SnakeGrid has been defined then its definition is fixed, and its use will be consistent with whatever coordinate accuracy is required.