As I mentioned, it’s not a good idea to think of the print position as having coordinates in the conventional sense. Jonathan’s tutorial is probably reflective of his peculiar paradigm. No need to rack your brain over it. Just remember that it’s “at row R, column C.” Forget the X and Y. Leave them strictly to pixel coordinates.
Another important aspect regarding coordinates on the Spectrum is that the graph in your picture is, well, also misleading. While you could set up an arbitrary origin point in your code (and, in fact, the BASIC PLOT routine does just that), the easiest and most natural way is to pin it to the top left corner. This way, increasing an address in the display file will also naturally increase the coordinates.