+ or -
Whether to choose the + sign or the - sign in
qv = qtarget +/- 90o where qtarget = tan-1 (y/x)
depends on which way the field is - which affects which way the force is.
We need to use the right hand rule to determine the direction of the force. We must then direct the velocity so that the force will bend the particle into the target rather than away from it.