For Ayu, it is random.
I believe the last time a tour extension happened, we had a finale at Osaka Jo Hall during the first set as well. Or it was slated before a Yoyogi was added to the end.
One thing though, Osaka Jo Hall is almost always just before the tour final.
http://monochrome0408.web.fc2.com/PC/LIVE/LIVE-MAIN.htm
Monochrome has been documenting her live #s and after this set of shows, she'll be at concert #507 with CDLs included.