I wrote this above;
(And yes, the variables at work with BER are very well known and I can give you a link if you want me to.)
I can't say there were lots of folks who asked me to provide that link, actually no one did,but I'm going to do it anyway since there's lots of information in it that hasn't even been mentioned here.
http://www.webgrower.com/information/carolyn_ber.html
Mike Dunton at Victory Seeds had asked me to write it, and I included the info from 2002 and updated other information.
And I'll update it even more right now.
I forgot to mention the difference between external BER, where you see that black area at the blossom end,from internal BER where there's no evidence at all that a fruit has BER until you cut it open and see all the rotted black areas.
I also left out info for folks who use Earthboxes and those who use only artificial mix to grow their tomatoes.
I can add that info if anyone wants me to do so.
I hope the above helps.
Carolyn