V1

Cos[θ + π/2] = -Sin[θ]

Sin[θ + π/2] = Cos[θ]

V1x = Dv1 Cos[θ]

V1y = Dv1 Sin[θ]

Vx == -Dv Sin[θ]

Vy = Dv Cos[θ]

By == (V1y - Vy)/(V1x - Vx) (Bx - Vx) + Vy

DvH1/Bz == ((V1x - Vx)^2 + (V1y - Vy)^2)^(1/2)/((Bx - Vx)^2 + (By - Vy)^2)^(1/2)

Solve