I've used the following formula before:
[Δ]v = 1.2Ph / (AEv)
where,
[Δ]v = horizontal shear deflection at the top of the wall
P = horizontal force at the top of the wall
h = vertical height of the wall
A = cross sectional area of the wall (in a horizontal plane)
Ev = Shear modulus (sometimes referred to as G)