I'm about to go to bed here pretty quick but here is my findings really quick.
Every item (food) has worth. We are going to be using this worth as our X value. Find the sum of your 'feed' see where your X term lies.
15 ≤ x < 50 50 ≤ x < 100 x ≥ 100
1 Guano 2 Guano 3 Guano
The maximum amount of Guano you can receive is 3 from one err.. load.
I will post my "research" backing this up. later.
An example is if I give 5 meat i get 2 guano.
While if I give 10 eggs I get 2 guano. Both have a value of 50.
Meat =10
Egg = 5
Will clarify if more explanation is needed. I'm off to bed have a test in the morning.