This is the 100 percent level. When we have improved the MTBF to this level, we shall have completely met our reliability requirement. If the impact of Big Idea was estimated at 900 hours, we would be making no forward progress toward our planned level. It is unlikely that anyone needs a new idea to get to where they are already. However, if the impact of Big Idea was estimated at 950 hours MTBF, it is a much more interesting design idea.
All estimates are uncertain.
It is useful to estimate how uncertain they are. This helps you understand the risk of not meeting desired goals. Evidence for Impact Assertion. If you want any credibility for your assertions, you should be prepared to supply facts to back them up. Instead of waiting to be asked, dig up the facts and document them in advance.