I don't have the reference handy. I'm pretty sure 9 was the trigger length, but the smallest chunk you could chop down to was the average rotation length. So, if the average was 6.3 days, you could theoretically move an X day into the middle of a string of 15 to create two 7 day blocks. But other rules (max x day groupings) could be the stopper against that. I think you can PD two days off the end of a 9-day string in this example though... could be wrong!