It is extremely important to achieve vertical position of the upper jaw in orthognathic surgery. A common method used is measuring distances at surgery using external reference points. Unfortunately, surgeons have been making mistakes when calculating the goal distance of our external reference systems. A possible explanation is that when the author measure the baseline and goal vertical distances, the author usually do not consider the variations produced by the triangulation effect generated due the horizontal or/and transverse movements of the maxilla. A method is proposed to fix this phenomenon in an easy way through the virtual customization of the ERP in computer-assisted surgical simulation. The technique entails creating, in the planning software, the same reference system one will use at surgery. In this virtual system, one measures the difference between the baseline and target distances, and then uses them at surgery.