My fitter recommended a 3mm shim under my left Look cleat to help offset a leg length difference. I'd like to get the same effect with my mtb shoes that use an SPD SH-51 cleat. As the cleat's surface is so small and that no shim is really available anyway, would I get the same effect by placing a 3mm shim inside the shoe, under the shoe's removable insole right over the cleat area? This is provided that there is enough room left in the shoe for my instep to be raised up in the upper.