i really hope it helps, but i suspect that if there is a BER issue that an application now will only affect those fruits which form from now on.
my experience with BER is that it sometimes happens on the first fruits but not the later ones (when the root system is larger and more able to keep up with demand - but also because it gets cooler).
the most simple solution (the one i like the most

) is to ignore it for us because it doesn't affect enough of the crop. this season we had no BER. we had pretty regular rains for the first month the plants were in the ground and all that cloud cover meant nothing was growing quickly up top but perhaps that meant the root systems were able to get larger than we'd normally see... can't say for sure.
we just started tomato canning season.