A fabrications facility is turning out directional couplers in an I/O planar geometry, in Ti-diffused LiNb03. The inter-waveguide gaps vary with fabrication conditions, but the following data were obtained from various guides: gap (in μm) 4, 5 and 6 and the corresponding complete coupling length (in mm) 7, 7.5 and 9, respectively. Calculate the length required for a 50% coupling when the gap is 7 μD.