Yes, we need to understand why you are trying to use snapshot delta in the first place..
Billyd, -
IMHO - Here is the way you should do it using the cmdlets from netapp
convertto-formattednumber ((get-navol vol0 | get-nasnapshot | measure -property Total -sum).sum) data
size "0.0"
4.5 GB
You can you system.math .net functions, but yours should look something like this
[math]::round(((get-navol vol0 | get-nasnapshot | measure -property Total -sum).sum/1gb),2)
4.5