chainstay flush with bb

Hi all,

I would like the chainstays to be as low as possible on my frames meaning that I want the lower part of the chainstay to come out flush with the bottom of my bb. I had a look in the chain stay section but I'm not quite sure how to work it out, i realise i can fill in a negative value for E and this will lower it, but I am not sure what E stands for. mm under or above center bb?

thanks, mick