Build A soil recommended watering 5-10% the volume of soil. An earthbox junior holds somewhere between 3.5-4 gallons of soil. So 5% would be between .17 & .2 gallons of water. 10% would be between .35 & .4 gallons of water. So a quarter gallon would be .25 gallons and would be right in the middle of that. I don't understand why you have to top water that .25 gallons... And why you can't just put .25 gallons into the res. If you watered .25 gallons into the res instead of top watering... Wouldn't you then have a better idea of how fast the plants are using the water... And then have a better idea of when to start filling the res completely?