Abstract: In the case of oblique exposure ellipse imaging of the girth welds of small diameter tube, the offset distance between the focus on the flaw detector and the plane of the weld centers should be calculated firstly. A chart for calculating the offset distance has been designed. The chart was composed of the curve graph and the nomograph. With the given parameter value, the precise figure of the offset distance can be obtained quickly by a ruler. The chart made the on-site testing work more convenient. Meanwhile, it can be used as a fast verification tool for quickly verifying the specified result in the operation instruction.