Calculation appropriate size to propose Quest DR Series

Hi anyone. Just want to some input from here. Current customer using a Netvault Backup Server which their store data for all backup inside a tape library with 3 drive. Now if we want to propose a DR Series appliance as their new target for store all the backup data, may i know any formulation that can be used to proposed the suitable capacity of DR Series? What the important report that we can generated from netvault backup server to help in calculation capacity need in DR Series. Hope anyone here can advise. Thanks