You're almost never wandering into complete unknown. Your original estimate already encompasses your uncertainty; padding it protects you from optimism.
Right but the padding is basically an estimate of how certain you are about your uncertainty. So not only does the original estimate not encompass your own uncertainty but it's completely arbitrarily scaled by multiples of the original estimate. Which is rather to say the original estimate is completely useless. Not to mention the OP is never more than 1/5 of this greatly inflated estimate off!!!
Whereas padding in a more assembly line like situation can actually be measured and forecast from historical data.
