Best thing I have found for plants susceptible to BER is, a low N, (in the case of tomatoes, a tomato specific plant food) fertilizer and adding calcium nitrate to the reservoir at the first sign of flowers. If I was using a high N fertilizer, I would use cal-mag instead. Calcium nitrate doesn't need microbial activity to make it available to the plant roots. The Espoma fertilizers have been working very well for me this year. When I remove the plants at the end of the season, there is this layer of resultant humus where the roots meat the potting mix, and there is this dense mass of fat, white root tips that look extraordinarily healthy.
Here was an experiment using both man made 10-10-10 (plant on the right), and Espoma Tomato-Tone (3-4-6). Both planted at the same time. The plant on the left has also has been topped 16" and it was around 9 feet tall. The 10-10-10 plant is definitely too much N for a closed system, IMO.
This below, was an orphan plant that lived in a solo cup for almost a month after the others were planted. I was curious to see if the natural fertilizer could rehab this root bound plant, that had grown to around 16" in a solo cup. I didn't have high hopes for it, but it is surpassing the man made nutrient plant and on it's way to becoming a monster as well, again, just using Tomato-Tone. It's kind of late in the season for it to be starting with fruit, but we will see. At least I got to see it turn into a healthy plant, and I have no doubts it would be a top producer.
The tomato tone plants are a normal, healthy green color, while the 10-10-10 plant's leaves are too dark and leathery, and the plant has had some intermittent BER as well.