The simulation result in INTERCONNECT definitely depends on FDTD result where is originated from the device configuration, eg, the design parameters. Usually you will need to optimize the coupling length, or the gap, for a given waveguide geometry/material. If the coupling is strong, you can either increase the gap, or decrease the length.
In order to get knowledge on how do the core width, thickness and slab thickness affects the coupling you can sweep them (looped sweep):
https://support.lumerical.com/hc/en-us/articles/360034922913-Creating-nested-parameter-sweeps but it will take long time since it is looped. This is your design process.
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