Direction for minimum circle
In circular motion, the force is perpendicular to the velocity. The force is directed toward the center, and the velocity is directed parallel to the circle (tangent).
In our case, to make the particle go in a circle of minimum radius to hit the target, we must choose a v to fit: r = mv/qB = d/2.
We must choose a direction that is perpendicular to the direction to the target:
qv = qtarget +/- 90o where qtarget = tan-1 (y/x).