IIRC it depends on the black hole. Right now there isn't a precise formula for it.
The breakup of the star would form an accretion disc, the jets would form at the axis of rotation. The formation of the jets are classically mechanical (conservation of angular momentum by various means) and so the focus of the jet depends on the speed of the matter being ejected, the amount of matter being ejected and the mass of the black hole.
Moreover, the matter falling into the black hole is heated to plasma level and so there is very strong magnetic activity. As the accretion disc spins, the magnetic field lines twist along the axis of rotation - the same axis as the ejected matter. This has the upshot of focusing the jets very tightly.
The breakup of the star would form an accretion disc, the jets would form at the axis of rotation. The formation of the jets are classically mechanical (conservation of angular momentum by various means) and so the focus of the jet depends on the speed of the matter being ejected, the amount of matter being ejected and the mass of the black hole.
Moreover, the matter falling into the black hole is heated to plasma level and so there is very strong magnetic activity. As the accretion disc spins, the magnetic field lines twist along the axis of rotation - the same axis as the ejected matter. This has the upshot of focusing the jets very tightly.
http://en.wikipedia.org/wiki/Black_hole#Accretion_of_matter