There's no "formula" for RNG. Moreover, I've done more than 200 runs of ifrit, 400 runs of garuda, and 50 (clears) of titan. I have never gotten the ifrit bow, yet seen every FC member of mine with it. Mind you, I've gotten both garuda and titan bows. In addition, I would run garuda and see a good variety of everything, then the next day 18 straight scholar books, while my FC scholar members complain about garuda never dropping scholar books in the whole 3 runs that they did. Definitely random.