Unfortunately, the mathematics points to "you need good judgement": http://www.idiom.com/~zilla/Work/Softestim/softestim.html http://www.idiom.com/~zilla/Work/S

Unfortunately, the mathematics points to "you need good judgement": http://www.idiom.com/~zilla/Work/Softestim/softestim.html http://www.idiom.com/~zilla/Work/Softestim/softestim.html At the very least, large limits exist as to how accurately one can estimate this sort of thing.