I corrected the positions for all the grid points using the best intersection point for each pair of lines. The average deviation of these points from the positions from the equation fit was nearly halved.
A more centrally located origin was also chosen and as a result the formulas required some additional modification.
Supplemental (3 Jun 2011): I haven't come to a final decision on "Zen" yet. Each intersection is determined by all the points in its two lines. But although the deviation of the improved points is about a half the measured positions it hasn't been shown that the procedure results in more accurate functions.