It is simply mathetically **wrong** to add anything but mean (P50) estimates also known as "proved and probable". Adding P90 estimates will give a number, but it is no longer a P90 estimate. Only P50 estimates can be added so that the total is still a P50 estimate and it is easy to understand why: In a set of P50 estimates half of the numbers will be overestimated and half will be understimated. The uncertainty will cancel in the sum leaving a total P50 estimate.