% complete reporting

On non-physical tasks, where there is nothing "tangible" produced as such, such as design tasks, engineering drawings, etc, it can be very difficult to effectivley measure progress. How can you say when something is 50% complete, 75% complete and so on. I have heard of some organisations adopting "milestones" within tasks and allocating specific % complete to those milestones. For example, when a drawing is issued for review it gets a certain % complete. I have never had first hand experience of this. Does anyone use any systems similar to this, or can give me pointers on how to better report % complete on these types of tasks.

Thanks,
Angus