The Build a Flower is good for several reasons. I like it for the life that comes with it. A good quality compost will have ingredients for veg and flower, and a lot Trace elements. Build a Flower is mainly for flower. The compost gives the plant more food.....period. It's just more cost-effective. You probably could just use build a flower. I don't think it would hurt anything, but the amount that you would need to feed in veg and flower would just too costly. You got to remember, you still need vegetative nutrients in flower.
Remember, you must have phosphorus in the soil before the microbes can extract it.
Just like biodiversity in your microbes and other garden helpers, it's good to have multiple sources of ingredients that the plant needs.
This run I'm using Coast of Maine lobster compost as my main compost and the Oly as a supplemental.75/25 ratio. I have several reasons for that, but cost is mainly the reason.......shipping and initial cost. Again, multiple sources of ingredients and biodiversity that's in the composts.
Remember one thing, you're growing autos and BAS recommendations are mainly for photos, as are most ingredient labels. I always go half and see the results.
Don't worry a whole lot on what you have in the nutrient Bank as far as the amounts. Once you get the plant on the reservoir and she's taking her water from there mainly, the feeder Roots can go into the new bank and get what nutrients they need at the moment, without being forced to take up the nutrients along with the moisture. That is the magic of Earth boxes. A very key point and why you don't want nutrients washing down into your reservoir.
This is one of the key concepts that you need to grasp in using Earth boxes.
Hope this helps!
earthboxes I don't know tuned into find out though 
