One of the major challenges of optical fabrication is measurement of the surface when first polished out, before its figure is within the capture range of interferometry. A high dynamic range instrument with good accuracy is needed to efficiently guide the processing. Our approach is to use the Software Configurable Optical Test System (SCOTS), a deflectometry technique that uses a camera and liquid crystal display to measure the surface slope. We describe the use of SCOTS in the fabrication of a 6.5 m on-axis mirror and an 8.4 m off-axis mirror segment (Giant Magellan Telescope primary). SCOTS has the dynamic range to measure high slope errors early in the mirror figuring process, with sufficient accuracy to corroborate later interferometric measurements. Accurately measuring low order figure errors with SCOTS requires careful calibration of the system geometry. Details of the data collection and processing, and comparison to interferometry measurements are presented.