When growing less than 12 plants:
Use 50ml of water per plant
1 drop of plant food for every 2 plants
Store the remainder of the SeedPods until you use them
A brief explanation:
The water will be stored in the reservoir (instead of absorbed by the 12 SeedPods and the reservoir) which is why you can use less water (proportionally) than the normal amount. This also helps reduce the possibility of algae growing.