I have never had difficulty in telling them the start and end point up front but don't give them time to try this out before telling them I want to do it sector by sector. In my experience, they are very happy if you offer to help them and know what you want - but if they tell you something is not possible then it is a matter of losing face so they don't want to back down. That's why it's best to help them avoid a situation which might need backing down.