Wow...21 inches is HUGE for a tutu, I don't think they ever actually run that big! Going with 12 layers was probably a good idea (the wider you go, the more rows you'll probably want to add on).
I've made two tutus in a year, and I really am itching to make Kreahe's anime tutu and finally have an idea of how I want to tackle it, so I may begin working on it and finish it whenever. I have a few other things I want to take priority for 2013 though so we'll see how it goes.
2014 Conventions :
Katsucon (Washington, DC) - Megacon (FL) - Colossalcon (OH)
Upcoming costumes : NiGHTS from NiGHTS: Journey of Dreams, Princess Kraehe (anime) from Princess Tutu, and Zidane Tribal from FFIX!