yes, you need to capture the selected value on to a token, which you can use later.
Also, as your search seems to be common, suggest, you use base and post-process search.
your base search can be
index=_internal idx="*" source=*license_usage.log type=Usage | eval totalMB = b/1024/1024 | eval totalGB = totalMB /1024
| rename idx as Index |stats sum(totalGB) as LicenseUsage by Index
Then in the post process search, you can use the token
where LicenseUsage > $token_value$
https://docs.splunk.com/Documentation/Splunk/7.2.4/Viz/Savedsearches
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