Hi, please be aware snap delta can consume alot of resources to calculate the difference between 2 snapshots - in 7mode. It would not be recomended to run this regularly.
You will need to create a new snapshot of the volume and perform the snap delta between that and the SnapMirror snapshot. This should show you the difference in KB between the snapshots if you were to initiate the snapmirror update now.
Referring to the example given on page 54 Data Protection Online Backup and Recovery Guide: https://library.netapp.com/ecm/ecm_download_file/ECMP1635994
The snap delta between nightly.0 hourly.1 is 6860KB as seen in the Summary...
We can achive the 6860KB figure by adding all the values in the KB changed column between the snapshots created between nightly.0 and hourly.1, seen in the first section of the output.
Also, if you have SnapMirror network compression enabled, run the command snapmirror status -l to see what compression ratio you have a achieved previously to give an estimate of the actual transfer size.
Hope this helps.