there are a couple of concepts you need to acquire before doing these calculations:
1- Veeam does backups at the block level, not the file level, so the size of the files in a volume may not be completely equal to the size of the backup.
2- we do compression and deduplication over a backup file, so the source size and the target size may be different.
This said, did you used by any chance the WAN accelerator for the backup job to the remote location? Because "transferred" means exactly data transferred from repo to repo, but it doesn't mean that some blocks were reused at destination by the destination cache of the wan-x for example...
for sizing, have you ever used the Restore Point Simulator?:http://rps.dewin.me/
It may help you in these calculations.