Did Miles actually design it? My understanding is that he was the head of a flag committee that was hip deep in designs submitted by the southern public.
Quite a few designs were submitted by various people. On 9 Feb 1861 William Porcher Miles was appointed head of the 'Committee on the Flag and Seal' by the provisional Congress. On 27 Aug 1861 Miles re-submitted his twice rejected proposal (originally submitted in February). Col. James P. Wilson of Louisiana submitted an almost identical design different only in that he advocated a Latin cross (St. George's) instead of Miles' St. Andrew's cross. Beauregard preferred the Miles' design. The committee then selected Miles' proposal as the official design.