Half of the flux ***
In the field of the moving charge Q, given by Eq. (5.15), we want to find an angle δ such that half of the total flux from Q is contained between the two conical surfaces θ′=π/2+δ and θ′=π/2−δ. If you have done Exercise 5.20 you have already done most of the work. You should find that for γ≫1, the angle between the two cones is on the order of 1/γ.