Tiny warm blooded lemmings are the primary consumers. A collared lemming weights about 40 grams (1.4 ounces).
I go to Some Animal Weights and Caloric Requirements on Earth. I see that a mouse living in a lab and weighing about 40 grams needs 20 Kilocalories a day. But this little mouse is not living in below freezing temperatures, is not running from predators, and is probably not reproducing frantically, as the lemming is. I will guess that a lemming needs double that, say 40 calories a day to live and metabolize. (This is purely a guess and may be very far off indeed!)
This lemming is going to be alive all year, and it will not hibernate. It needs to eat every day. So 40 Kilocalories a day for 365 days means that the lemming will need about 14,600 Kilocalories in a year.
So how many square meters does the lemming need for a year?
14,600
Kilocalories = 600 Kilocalories per square meter per year. 
24.3 square meters I put in 25 to make the math easier. 
From this you can see how much space your predators need.
If an owl eats 12 lemmings a day, and each lemming needs 25 square meters a year, how many square meters does it take to support an owl?
How many owls can live on a square kilometer?
Note that the predator numbers here are about the animals that get eaten, not about the community of animals necessary to keep producing the animals that get eaten. As a guess, I would suggest using only half or even a third of the predators per Kilometer to allow for a continuing producing community.
Lemmings reproduce rapidly. If you are doing this with wolves and elk (large animals that produce one calf annually) I would guess that wolves could eat only 5 to 10% of the elk herd without driving it to extinction.
Tiny warm blooded lemmings are the primary consumers. A collared lemming weights about 40 grams (1.4 ounces).
I go to Some Animal Weights and Caloric Requirements on Earth. I see that a mouse living in a lab and weighing about 40 grams needs 20 Kilocalories a day. But this little mouse is not living in below freezing temperatures, is not running from predators, and is probably not reproducing frantically, as the lemming is. I will guess that a lemming needs double that, say 40 calories a day to live and metabolize. (This is purely a guess and may be very far off indeed!)
This lemming is going to be alive all year, and it will not hibernate. It needs to eat every day. So 40 Kilocalories a day for 365 days means that the lemming will need about 14,600 Kilocalories in a year.
So how many square meters does the lemming need for a year?
14,600
Kilocalories = 600 Kilocalories per square meter per year. 
24.3 square meters I put in 25 to make the math easier. 
From this you can see how much space your predators need.
If an owl eats 12 lemmings a day, and each lemming needs 25 square meters a year, how many square meters does it take to support an owl?
How many owls can live on a square kilometer?
Note that the predator numbers here are about the animals that get eaten, not about the community of animals necessary to keep producing the animals that get eaten. As a guess, I would suggest using only half or even a third of the predators per Kilometer to allow for a continuing producing community.
Lemmings reproduce rapidly. If you are doing this with wolves and elk (large animals that produce one calf annually) I would guess that wolves could eat only 5 to 10% of the elk herd without driving it to extinction.



KCal Needed Per Day 



Total Numbers of Square Meters Needed Per Year  Square Kilometers needed per year (divide by 1000 x1000) 
Animals per Square Kilometer 
lemming  plants  40  14,600  600 

25  .000025  40,000  
Snowy Owl  12 lemmings 
300 
109,500 
.11  9  
Arctic Fox  30 lemmings 
750 
273,750 
.27  3  
However, it is obvious that if a predator's range (feeding area) is does not yield enough food, the predator will starve. If a predator has to spend more Kilocalories going to find food than is in the food he/she finds, the predator cannot live there.
There is also the complicating factor that the predator has to find a mate, and also find more food to feed his/her young.
The predators are the arctic fox (5 10 lb) and the snowy owl. 2 4 pounds (eats about a dozen lemmings a day)
1 0z meat = 100 Kilocalories
Life Form  Needs ? KCalories per Day  Needs Calories per Year  Needs ? Square Meters per Year at 600 Kcal each  weighs  Caloric Content per Gram 
Caloric Content if Eaten 3 Kcal * weight in grams 
Calories per year Calories if eaten 
lemming  40  40 *365 =14,600  25  40 grams  3 Kilocalories  3*40 = 120 Kcal 
121.67 
Snowy Owl  12 lemmings @ 120 Kcal= 1440 
1440 * 365 = 525,600 Kcal per year 
12 lemmings * 25 sq meters * 365 days = 109,500 sq meters 
2  4 lbs 3 lbs = 1,361 grams 
3 Kilocalories 
3* 1361= 4,083 Kcal 
128.73 
Arctic Fox 
30 lemmings (?) * 120 Kcal =3,600 
3600 * 365 = 2,394,000 Kcal per year 
30 lemmings * 25 sq meters * 365 = 273, 750 sq meters 
5  10 pounds 7 pounds = 3,175 grams 
3 Kilocalories 
3*3175 = 9,525 Kcal 



KCal Needed Per Day 



Total Numbers of Square Meters Needed Per Year  Square Kilometers needed per year (divide by 1000 x1000) 
Animals per Square Kilometer 
lemming  plants  40  14,600  600 

25  .000025  40,000  
Snowy Owl  12 lemmings 
300 
109,500 
.11  9  
Arctic Fox  30 lemmings 
750 
273,750 
.27  3  
However, it is obvious that if a predator's range (feeding area) is does not yield enough food, the predator will starve. If a predator has to spend more Kilocalories going to find food than is in the food he/she finds, the predator cannot live there.
There is also the complicating factor that the predator has to find a mate, and also find more food to feed his/her young.
The predators are the arctic fox (5 10 lb) and the snowy owl. 2 4 pounds (eats about a dozen lemmings a day)
Life Form  Needs ? KCalories per Day  Needs Calories per Year  Needs ? Square Meters per Year at 600 Kcal each  weighs  Caloric Content per Gram 
Caloric Content if Eaten 3 Kcal * weight in grams 
Calories per year Calories if eaten 
lemming  40  40 *365 =14,600  25  40 grams  3 Kilocalories  3*40 = 120 Kcal 
121.67 
Snowy Owl  12 lemmings @ 120 Kcal= 1440 
1440 * 365 = 525,600 Kcal per year 
12 lemmings * 25 sq meters * 365 days = 109,500 sq meters 
2  4 lbs 3 lbs = 1,361 grams 
3 Kilocalories 
3* 1361= 4,083 Kcal 
128.73 
Arctic Fox 
30 lemmings (?) * 120 Kcal =3,600 
3600 * 365 = 2,394,000 Kcal per year 
30 lemmings * 25 sq meters * 365 = 273, 750 sq meters 
5  10 pounds 7 pounds = 3,175 grams 
3 Kilocalories 
3*3175 = 9,525 Kcal 