If I had that configuration and was only interested in keeping 30 days of backups, I'd honestly probably just use reverse incremental and not use Windows 2012 dedupe at all. Veeam with normal compression will likely redeuce your backup to <2TB, and your incrementals to <300GB on average. That means that 30 days of backups would only need ~11TB anyway, and you said you have a 16TB volume, and that's assuming only a 2:1 compression ratio, most environments see closer to 3:1 which would out it closer to 9TB with Veeam alone.
On the other hand, if you just want to use Windows 2012 dedupe and forward incremental, it sounds likes it's already working reasonably well (it's saved 4.29TB). Do you have all 5 full backups yet? That's where most of the dedupe will come from. Dedupe friendly compression will reduce your dedupe ratios by quite a bit and normally I would suggest using no compression at all if you are interested in getting the absolute best dedupe ratios.