  We develop an improved phase calibration method of a reflective spatial light
modulator (SLM) using interferometry by employing novel phase masks. We
generate the optimised phase masks by using Iterative Fourier Transform
Algorithm (IFTA) and demonstrate that they perform with 18$\%$ better
efficiency over global linear corrections in the look-up table (LUT). In
addition, we also implement global linear corrections in the LUT and
correspondingly modify the phase encoding of blazed gratings to improve the
efficiency of our phase-limited SLM. In the process, we definitively determine
the actual maximum phase throw of our SLM which provides a recipe for users to
verify supplier specifications.Besides obtaining array of 1D/2D spots having
high uniformity (~90$\%$) using IFTA, our result exemplifies the use of
iterative algorithms for improving efficiency of phase limited SLMs. Finally,
our method enables threefold faster phase measurements, and to the best of our
knowledge, is the first endeavour directed towards enabling rapid phase
characterisation of an SLM using interferometric measurements and can have very
useful applications in settings which often require faster phase calibrations.
