Depends on your recipe. Some recipes, like those in Palmer's "How to Brew", give 2 OGs: one for 3 gallons of wort, and one for 5 gallons of wort.
Why not just take 2 measurements?
Fermenter 1 - Vienna /Saaz SMaSH
Fermenter 2 - Dry as a bone
Drinking - various craft brews, Tiny Bottom PA
Beer styles I'm trying to nail down: APA, Porter, Mild, Amber, & Something Yellow and Fizzy.