When zeroing with a tool to the edge of your stock, you need to compensate for the radius of the tool. So, for instance, if you were using a half-inch tool touched off to the edge, you'd add a quarter inch if it was on the negative side, or subtract it if it was on the positive side. If your controller doesn't let you put in any value but zero, move the tool up to clear the stock and a quarter-inch over to the edge, then press the zero button. Usually it's recommended to do this with an edge-finder instead of the actual tool, to avoid damage to the tool and/or the stock.