Aside from using calculations; your FG would typically be...
whatever the gravity is when you check your hydrometer readings a few days between and it is unchanged....
Based on yeast, temperature, etc. (and, really, your recipe should list a FG ballpark, no?).
Good luck and enjoy the final product!
Secondary: BrewHouse IPA
Forgotten about: Samichlaus
Keg: Special Bitter
Keg 2: American Barleywine