orient a circle under an object?

I would like to orient a 2D circle underneath or aligned a 3D Object as it appears that the circle is a base to the to hemisphere.
Kindly find the attached picture for more clarification.

I use the following code that gives wrong orientation.

    gl::pushMatrices();
     Quatf q;
     Vec3f dir = m_tentacle[0]->geNodesPos()[0] -  m_tentacle[0]->geNodesPos()[8];
     q.set(Vec3f(0,-1,0),Vec3f(dir.x,dir.y,dir.z));
 
     gl::translate(m_SpherePos);
     gl::rotate(q);
 
     drawHemiSphere(m_iRadius,90);
 
     gl::popMatrices();
 
     gl::pushMatrices();
 
     Vec3f axis  = q.getAxis();
     float angle = q.getAngle();
     Quatf incQuat(dir,angle);
     gl::translate(m_SpherePos);
     gl::rotate(incQuat);
     gl::drawStrokedCircle(Vec2f(0,0),100);
 
     gl::popMatrices();
 

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