A ski shop actually mounted the bindings based on you telling them you are a size 12? Do NOT go back there!
Like qcanoe said, you need to use the boot you will be skiing in to mount the bindings correctly. Ski boot sizing does not exactly relate to shoe sizing. Street shoe can have some limited correlation to the mondo size (shell size), but the critical measurement is the boot sole length (BSL). This is a measure in mm of exactly how long the boot is from toe to heel. The BSL along with the boot center line determine where on the ski the binding is finally mounted and the correct spacing between the toe and the heel. Some adjustment is possible but in order for the boot to release properly from the binding, the forward pressure setting must be set within the correct range. If any adjustment is made to the heel location or forward pressure setting, the DIN settings will need to be re-checked as well.
Bottom line, if you have moved the heel piece all the way forward and when the boot is "latched in" there is a 1/2 inch gap, you will most likely need to have the bindings re-mounted. However, I would suggest taking the boots and skis to a reputable shop and have them check it out.