Gpw doesn't really work with trees, it's more weight per plant as usually people growing trees have to because of low plant numbers. 3lb per plant for your first tree grow is insane, I'm sorry it's not gonna happen, especially in dirt. Hope for 1 per plant and go from there. In this business getting your hopes up for a huge yield then having reality set in when you crop a 1/4 of what u expected is crushing.. Ask any experienced grower and they will agree. 20 gallons of dirt is plenty, I've yielded half and 3/4 of a lb in 5 gallon buckets of dirt. Also if you wanted to fill up a 30 gallon bucket with roots u would need about 3 months veg time, 10 ft ceilings and about 50sqft for each plant.