Something like this?
Select Top 1000000
tblDiskdrives.Caption,
tblDiskdrives.Freespace,
Cast(tblDiskdrives.Freespace / tblDiskdrives.Size * 100 As Numeric(36,2)) As [% Free],
-- base 2 gigs (memory is measured in base 2)
Cast(tblDiskdrives.Freespace / Power(2, 30) As Numeric(36,2)) As [GiB Free],
Cast(tblDiskdrives.Size / Power(2, 30) As Numeric(36,2)) As [GiB Capacity],
-- base 10 gigs (hard drives and such are marketed in base 10)
Cast(tblDiskdrives.Freespace / Power(10, 9) As Numeric(36,2)) As [GB Free],
Cast(tblDiskdrives.Size / Power(10, 9) As Numeric(36,2)) As [GB Capacity],
tblDiskdrives.Lastchanged As [Last changed],
tblAssets.AssetID,
tblAssets.Description,
tblAssets.AssetName
From
tblDiskdrives
Inner Join tblAssets On tblAssets.AssetID = tblDiskdrives.AssetID
Inner Join tblOperatingsystem On tblAssets.AssetID = tblOperatingsystem.AssetID
Inner Join tblComputersystem On tblAssets.AssetID = tblComputersystem.AssetID
Where
tblComputersystem.Domainrole > 1
And tblDiskdrives.DriveType = '3'
-- drive capacity is not zero MebiBytes
And Floor(tblDiskdrives.Size / Power(2, 20)) <> 0
-- available space <= 20%
And tblDiskdrives.Freespace / tblDiskdrives.Size * 100 <= 20
-- CHOOSE ONE:
-- EITHER (base 2 gigs free < 25)
AND Floor(tblDiskdrives.Freespace / Power(2, 30) ) < 25
-- OR (base 10 gigs free < 25)
AND Floor(tblDiskdrives.Freespace / Power(10, 9) ) < 25
Order By
[% Free],
tblDiskdrives.Freespace,
tblDiskdrives.Caption
I simplified some of the calculations -- there's no need to convert size/capacity to MB/GB when calculating percentage, for example. If I read what you're asking for correctly, you're just wanting to add one or the other of the FreeSpace < 25 statements.