The first thing to do is delete any snapshots that you don't want in the calculation and give the system time to free the disk space. This matters if you have a 6 month old snapshot the DBAs left there after an upgrade.
You know how much data changed in the volume from "df -k vol_name". You don't know if it is growth or churn from "df".
The "snap delta" tells you the rate.
Why not post here the output of "df -k vol", "snap list vol" and "snap delta vol". I'm sure several of us will give you some (contradictory) answers.