How much cleavage?

So how much cleavage needs to be on show before you are described as being slutty? For some ladies out there, it is virtually impossible to hide unless you are fully wrapped up in a polo neck.

So if the old rule of "less is more" still stands and you should only expose either boobs or legs - then how much boob / leg should be on show before you become a tart? Being "lucky" enough to have cleavage that seems to have a mind of its own, I can start evening perfectly respectable and then make a complete tit (yes pun intended) by the end of the night!