As discussed on this List in the past, you can't get client file sizes
from server queries, so there is no way therein to sum the savings of
changing retention policies.  You could approach it on the basis of
client queries on Inactive files.  Another approach is proportion: if
you have a good idea of the population of Inactive files relative to
total possible population per your retention policies, then reducing
the policy number by a certain percent would result in a proportional
percent drop in your current server storage pool numbers.

  Richard Sims, BU