It depends on which film do you use. If you want a very reduced DOF in direct bright sunlight, it is keeping the lens full open or close to (f5.6 or lower), use an ND filter and readjust the ISO setting in accordance with filter factor. That´s because the meter cell is in the camera´s front, not within the lens ring and doesn´t compensate automatically for filter factors as some other cameras do (Canonet, Yashica MG1, etc.).
Never forget to readjust ISO setting after filter removal.
Ernesto