Tsm v 7.1.7.1 running under RHEL 6.8.

Trying to fill out a vendor's questionnaire regarding backup.
They want an estimate of the size of a full backup of our environment.
Given the infinite incremental nature of tsm, will the following script work?
I am also open to a better suggestion if it exists.
select node_name, -
sum( capacity* pct_util/ 100/ 1024 ) as "used GB" -
from filespaces -
group by node_name -
order by node_name

thanks for the help.