Jyoti Mahato I'm afraid that it would be virtually impossible to accurately model the flow around the fishbone spindle. This prevents the determination of its geometry factor and thus the conversion that you are looking for. Even for the vane tool - which is more simple and way more usual - this determination is not achieved; you may find this information in rheometers' manuals.
Alternatively, you may empirically make this conversion by analyzing some fluids that you know the rheological properties with your fishbone spindle. However, once again, those will not be the actual rheological properties, only estimated values.