In every project or task there are always things that get overlooked, don't go quite right, or are just wrong especially if you are moving fast or entering uncharted territory as often happens in a start up. I view this as the" 2%." The key is isolating this 2% down to things that will have the least material impact if they occur or, if they do, can be quickly fixed with limited impact.
Drive for perfection but understand it is elusive. Understand that things will go wrong but focus on keeping their impact to a minimum.
Venture Vice 
Interesting thought. Didn’t the Bush administration (and I think this was widely attributed to Cheney) use a 2% likelihood as the threshold for whether they threw full resources into a potential threat? I think you need fairly substantial resources to pursue this, but interesting, nonetheless.
Also, I think I’d express it like this:
Projects come in waves (phases), and each wave will have Pareto’s law (80/20) applied to it.
If you work this right, you leave 20% on the table at the first wave (e.g. private beta), but come back and get 80% closer on each wave (as you add features, these typically start at zero).
Three successive waves (e.g. private, then public beta and production rollout) should put you at something like 98% (assuming some added features).
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