To go even further on that point - the chart itself seems overly optimistic. +-40% of estimate after product design? +-10% of estimate after design spec?! Count

To go even further on that point - the chart itself seems overly optimistic. +-40% of estimate after product design? +-10% of estimate after design spec?! Countless number of times I have seen detailed design specs completely thrown out of whack by new engineering, process or business revelations/insights. Human thought is iterative - most people can imagine at most 80% of the future/product/design possibilities.