+1 on everything Thomas just said.
Also, remember that agitation methods and frequency have everything to do with contrast in developing film. As suggested, take off 10% - 20% off of your own time and, using your own technique, work your way down to the right time. If you are getting good even development with 120 film and Rodinal, I would keep the agitation scheme the same.
You could even blow a whole roll on one shot, cut the film in half, and try two times (new developer each time). Even if not perfect, you should be able to interpolate.