For a humbucking pickup, the area at which the string affects the pickup is quite large, so movement of the pickup just results in a tonal change (closer to the bridge = brighter, thinner), rather than there being a null point at a node of the string. Pressing a fret down will change this node point anyway, so even if you mathematically worked out the optimum position for an open string, this would only work for 1 particular note on the instrument.
I wouldn't worry too much about pickup placement-- if it looks right, then it probably is ;)