If the coolant is down 10-12 inches then I would fill it back up to the top. Drain the overflow tank to where its 1/3 full. Crank the car up and let it sit till the thermostat opens, if everything is the same then, take the car for a little drive. DO NOT go far, just down the road and turn around and come back. Let the car cool down and open the cap and see if the radiator is still full. If it gets to the "R" in Normal by the stock gauge, I would pull over and allow it to cool down, even though its a stock gauge you still dont want to take a chance over something that could have been prevented.
Dave
Dave