You might want to shoot a strip of 6 frames of the same scene with the same exposure settings. After developing & bleaching them together, cut the strip into individual frames. Redevelop first frame in FD-72 for 1, second for 2, third for 4, fourth for 8, fifth for 16, and sixth for 32 minutes. Compare the results and based on your subjective evaluation, fine tune the time further.
For Kodak's reversal movie film, the recommended redevelopment time was 8 minutes iirc.
Wear gloves.