My concern is the reciprocity. From what little I can find, Super-XX had almost 0 reciprocity effect. Knowing the reciprocity characteristics of the old film would help target a proper exposure for the film I'm using now.
Super-xx was rated at 100, that's before the big switch, though. So the first part is calculating the speed difference, whether or not to use the stated speed or the post-change speed. After that is the reciprocity. I don't see why it wouldn't work, and do so quite reliably, once those 2 bits were known.
Practicing with my digital camera may provide some help dialing in a chart for today's speeds, as settings are controllable and the feedback is quick, although I'd like to hear from someone who's tried it rather than resort to my other camera.