High performance in normal incidence soft X-ray optical systems requires accurate control of the d-spacing across the surface of each mirror in the system. As a first step towards being able to fabricate any desired d-spacing variation, we demonstrate the ability to produce large (25 x 150 mm) flat Mo/Si multilayer coated mirrors with a d-spacing uniformity of +/- 0.4 percent. Instead of applying the approach most often taken to minimize the d-spacing variation physical shielding of the deposition source, we use a mask with a corrected profile positioned just in front of the rotating substrate to compensate for the nonuniform deposition flux. Results obtained from hard (lambda = 0.154 nm) and soft (wavelength of interest) X-ray mapping of the surface are presented along with a discussion of the technique used to control the d-spacing distribution.