It's actually not my own rule. In the OSR community it's known as "Shields shall be splintered" and it's a common house rule to make shields a one shot negation of an attack. Then again, the shield also usually offers a passive bonus as well but I didn't want to ramp up the armor too much. I might give it 1 def and 3 uses but with the option to shatter to completely negate an attack.