An accurate mechanical method and associated calliper for determining the vertical and longitudinal center lines and the mid-point between joint holes in the laying out of a stirrup for use in the foundation of a lower extremity orthosis. The calliper comprises an assembly with three main parts: a floating, calibrated main bar containing two lateral internal springs of equal tension connected to a center punch or marking pin, and two end bars of equal size connected to the other ends of the internal springs, which end bars slide in and out of the main floating bar. Each end bar includes a pin which attaches it to the main bar spring, a stirrup dowel and a scale to be used for inter alia measuring varus or valgus deformities of the ankle. The assembly is placed on a stirrup by inserting the stirrup dowels on the two end bars into the joint holes on the stirrup, thus positioning the main bar for measurement and marking purposes. The center point is marked by, for example, striking the center marking pin with a hammer, which center point is used to determine the bend lines for the stirrup which are then used for appropriately bending the stirrup into the configuration desired for a particular user. The pins on the end bars can also be used if desired for marking the defining points of the stirrup's vertical or lateral axis.