You stated in your previous post that you had a length of heavy duty aluminium 1500mm long and an acurate machined surface that was straight to within 0.004mm if memory serves, use this to adjust your rail, i did explain how to do this some time ago as this question has been asked countless times, start fixing the rail from the center work your way alternately towards each end, you will then in your case have a section of rail 1500mm straight to the given accuracy of your reference gauge, you then use your secured section of rail as a datum for setting the remaining unsecured section of rail.