Good quality, richer, denser food: An 80 lb sedentary lab should eat about 3-4 cups. If he's active, he should eat more. A 12 lb Jack Russell (assuming he's the fairly energetic type) should eat about 1/2-1 cup per day. Mid-poor quality food with a lot of fillers will require more food. I know some foods that recommend 6-8 cups for an 80 pound dog.

You really just have to check the KCal/cup. That pretty much dictates how much you'll have to feed. Higher quality foods tend to to have more KCals/cup (i.e. Innova has something like 500 KCal/cup) whereas lower quality foods have less (i.e. Pedigree has something like 300 KCal/cup). To meet your dog's requirement, therefore, you'll have to feed more of the low-qual stuff.