I think you're painting it a bit too black. Having both sides agree to the fact that it's an estimate (and not an exact prediction) does help. And if you make the client understand the contingencies and unknown factors, you may get around the expectation to make an accurate prediction — which nobody can give anyway, except when doing the same thing the umpteenth time. Communicating often on progress that you actually do make can still give the client a good sense of confidence and control on what's happening.