If you have ran the strain before and know how long to run it, just pro-rate the corresponding grow week with the back of the bottle. Just do a simple proportion like x days actual / recommended. So in your case: 11 weeks / 9 weeks = 1.22. Multiply this number by whatever week you are feeding. For example, Week six of box nutrients would be 6 X 1.22 = 7.32 Weeks. So you start the ripener at 7 weeks and 2-3 days or so when running the Sour.
I'm sure you came to the same conclusion, but I figured I'd show you how I do it the nerdy, but simple way lol.