Once your seeds germinate, the first set of leaves they have are known as Cotyledon's. These leaves contain enough food for your seedling for roughly two weeks, and it is at 2 weeks of age your plants leave the seedling stage, and are then in the first week of Veg. I recommend starting feeding however after the first week of germination.
Like people, plants need both food, drink, and oxygen to not only thrive, but survive too. The nutrients is the plants food, the water is there drink, and the oxygen is sourced by them through the roots, and because of that, overwatering can kill your plants. Because of this, it is important to figure out how much water your plants can take. Once you have done that, you can then work out how much of the nutrient to add. You don't have to mix up a gallon and add the recommended dose for a gallon, you can do the math and change the volume of liquids you add to match the amount of water you intend to give.
If for example the guidelines say to add 20ml per gallon, half a gallon would be 10ml, and a quarter gallon would be 5. You can of course dissect this even further.
Hope this helps!
