I have reviewed Reed's video on coring. He also talks about and shows how to adjust the curvature of the blades. My question is on the "flatness of the blade". Some of my blades are completely parallel when resting on a flat surface. On others you can see some air under the middle of the blade where it is not touching. Should the blade be level to a flat surface for the entire length of the blade? If yes, how do you make that adjustment. (Hit it with a hammer?) Thanks