I would think that anyone selling wedges should have the tool to determine if/how many you would need.
It's simply a matter of measuring the "cant" of your foot in a neutral position. It takes minutes to do.
It is NOT a matter of just putting a lift under the heel. The wedges will have more material under the varus (inside) or valgus (outside) of the ball of your foot depending on your individual makeup.
"....No longer easy on the eyes but these wrinkles masterfully dusguise the youthful boy below."~Death Cab for Cutie
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