Just a word of warning, RNG can be a real pain. The first materia will be guaranteed but everything after that, you have a chance to fail and lose the materia, which can get quite expensive. I remember one of my rings, I needed a Crit III in my second slot. Easy enough right, high percentage chance at 35%? Took me NINETEEN tries to get it in. Figure Crit III materias is around 30-40k each and you can see why I was a very sad panda.