80hz is usually the best starting point not only with bookshelf speakers but with a lot of tower speakers if you have a well designed subwoofer. There is nothing wrong with trying 100hz. However, the further you increase beyond 80hz the subwoofer starts to become more and more localized. A higher crossover point can work and be beneficial with careful positioning of the subwoofer(s) close to the mains.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne