I usually put a coin on he ground and take four photos above it. You will find a small coin at the nadir of my swingrod panoramas.
Angles of elevation and depression should be determined in order to maximize the overlap between the fields of view.
Usually the nodal point of circular fisheye is at the center of the lens rim circle. The magnetic coupler is therefore adhered on the rim, but the center of the swingrod does not come just below the nodal point because of the angles of elevation/depression. To rotate the camera around the true nodal point, You must not rotate the camera around the swingrod. This photograph illustrates how to determine the offset. The offset distance of the rod end from the center of the positioning coin (true pivot) should be r sin A, where r is the radius of the rim and A is the angle of elevation.
In my Peleng fisheye's case, offset is about 1cm. Coin diameter is used as the guide for eyes.