Q. When I specify the coordinates of the nodes in my Field, do I need to specify the coordinates in x first, then y, then z?

A. The order in which you specify your coordinates doesn't matter so long as it is in the same order in which you specify the Field's dimensions and node data. If, in your dims array, you specified the dimensions in z, then y, then x, you have to provide the coordinates of the nodes in the same order and likewise, the values in the node_data.values array.