I'm not sure why you'd want to manage by FileGroup, but... What if you set up the same process that I outlined, but used this query to gather the data. I think it'd do what you want.
SELECT fg.GroupName
,SUM(f.size / 128.) AS TotalSizeInMB
,SUM(CAST(FILEPROPERTY(f.NAME, 'SpaceUsed') AS INT) / 128.) AS SpaceUsedInMB
,SUM(f.size / 128. - CAST(FILEPROPERTY(f.NAME, 'SpaceUsed') AS INT) / 128.) AS FreeSpaceInMB
FROM dbo.sysfiles f
JOIN [dbo].sysfilegroups fg
ON f.groupid = fg.groupid
GROUP BY fg.GroupName
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