Yeah, I do my own soil. High brix numbers in a plant are in the 20's. You measure how light refracts through your plants sap to get this number. So you'd take a few fan leaves and crush them up to get some sap and then measure it. Higher teens are pretty high numbers too. Bottles just tend to have set ratios and plants can vary a fair amount in what they really want at any given time. They often make do with something other than - Nitrogen can be used in a pinch for less effective nutrient use by the plant, they are pretty amazing in their ability to adapt - but it's not ideal for them.
Check out the RLOS thread in the organics, I know some of this stuff is covered. It's a really great way to garden and IMO is so much easier. You can get great brix levels with bottles too, it just requires a lot more dedication and experimenting.