Crossover Frequency

I remember awhile back someone had asked about crossover frequency and there was a way to determine what it should be set at based on the speaker. After looking up and down I have not been able to find the thread again. My upgrades are starting to come in and I'd like to optimize my system. Being a new heeled audiophile I could use a little guidance on this matter. Thanks guys and gals!
Am I to assume you are looking for help setting up a sub to match your mains?

If so, use the archives and enter keywords. I found this using "subwoofer setup"...
Thank you!