How does pressure balance work in the magnetosphere?

[ergospherical]

Homework Statement

How does the radius of the magnetosphere of a neutron star, accreting from a binary companion, scale with the surface magnetic field $B$, the radius $R$ and the accretion rate $\dot{M}$?

Relevant Equations

Spin down rate $\dot{\Omega} = -B^2 R^6 \Omega^3 / (I \mu_0 c^3)$

I'm not sure how to set up the pressure balance at the magnetosphere? Maybe some energy equation relating $\frac{1}{2} \dot{M} v^2 \sim$ magnetic energy density?