Hello Abhinav,
The modal overlap functionality calculates the power overlap between the fundamental input mode with the field at each slice during the calculation. Ultimately this will only give you the overlap between the output waveguide and the input mode. While the taper geometry will impact the quality of the field and therefore affect the overlap calculation it will be ultimately limited to the overlap of the fundamental mode of the input and output waveguides which is not what you want.
A better solution would be to build your design using parameters so that changing the “TaperLength” parameter will changed all of the other positions of the waveguides and then do a parameter sweep either using the parameter sweep functionality. As an example I am attaching a design where a 10um input waveguide is coupled to a 1um output waveguide through a taper and it is run with a script that scans the length of the taper from 500um to 4000um and exports the power within the output path. As you can see in the attached image (generated externally from the data exported by OptiBPM) as the coupling efficiency of the taper increases as the taper gets longer.
NOTE: The bpd file was generated with OptiBPM 13.1.1 and will require that version of the product to open it. I you are unable to open it disucss support options with support@optiwave.com
Scott