The Joint European X-Ray Telescope, JET-X, is one of the core instruments of the scientific payload of the Russian SPECTRUM-X astrophysics mission due for launch in 1995. JET-X is designed to study the emission from X-ray sources in the band from 0.3 to 10 keV, particularly to meet primary scientific goals in cosmology and extragalactic astronomy. JET-X consists of two identical, coaligned X-ray imaging telescopes, each with a spatial resolution of 30 arcsecond (Half Energy Width, HEW) or better. Each telescope is composed of a nested array of 12 mirrors with an aperture of 0.3 m and a focal length of 3.5 m. The mirror shells have Wolter I geometry and are replicated by an electroforming process for which Carl Zeiss manufactured the 12 monolithic Nickel coated aluminum mandrels. In order to determine the mandrel limited HEW, several measurement and analysis steps including raytracing calculations are performed. The major contributions to the error budget, axial slopes and roundness errors, as well as the position of the focus are investigated. The results are reported and discussed.