Systems Diagnostics - Database Sizes math?

Does anyone know how Used (GB) and Unused is derived? Maybe I'm wrong, going to assume it's looking at total_size, total_used, reserved_pages, ect.. Trying to figure out the math, and questioning if it's correct. This is what I have so far, any feedback is appreciated!

Dim sql As String =
        "SELECT " &
        "    DB_NAME()                                                                   AS DatabaseName, " &
        "    CAST(f.total_size      * 8.0 / 1048576 AS DECIMAL(10,2))                    AS TotalSizeGB, " &
        "    CAST(f.total_used      * 8.0 / 1048576 AS DECIMAL(10,2))                    AS UsedByFilesGB, " &
        "    CAST((f.total_size - f.total_used) " &
        "                           * 8.0 / 1048576 AS DECIMAL(10,2))                    AS UnallocatedGB, " &
        "    CAST(s.reserved_pages  * 8.0 / 1048576 AS DECIMAL(10,2))                    AS ReservedGB, " &
        "    CAST(s.data_pages      * 8.0 / 1048576 AS DECIMAL(10,2))                    AS DataGB, " &
        "    CAST(s.index_pages     * 8.0 / 1048576 AS DECIMAL(10,2))                    AS IndexSizeGB, " &
        "    CAST((s.reserved_pages - s.data_pages - s.index_pages) " &
        "                           * 8.0 / 1048576 AS DECIMAL(10,2))                    AS UnusedGB " &
        "FROM ( " &
        "    SELECT " &
        "        SUM(size)                               AS total_size, " &
        "        SUM(FILEPROPERTY(name, 'SpaceUsed'))    AS total_used " &
        "    FROM sys.database_files " &
        ") f " &
        "CROSS JOIN ( " &
        "    SELECT " &
        "        SUM(a.total_pages)                                           AS reserved_pages, " &
        "        SUM(CASE WHEN p.index_id IN (0,1) " &
        "                 THEN a.used_pages " &
        "                 ELSE 0 END)                                         AS data_pages, " &
        "        SUM(CASE WHEN p.index_id > 1 " &
        "                 THEN a.used_pages " &
        "                 ELSE 0 END)                                         AS index_pages " &
        "    FROM sys.partitions p " &
        "    JOIN sys.allocation_units a " &
        "      ON p.partition_id = a.container_id " &
        ") s"

Here's the output

--- Level 1 (Total) --- TotalSize: 1394.27 GB

UsedByFiles: 228.51 GB

Unallocated: 1165.75 GB (83.6% of Total)

LogFile: 1.67 GB (0.1% of Total)

--- Level 2 (Reserved) ---

Reserved: 226.85 GB (16.3% of Total)

UsedGB (Data+Index): 221.60 GB (15.9% of Total)

Unused: 5.25 GB (0.4% of Total)

--- Level 3 (Used) ---

Data: 129.73 GB (9.3% of Total)

IndexSize: 91.87 GB (6.6% of Total)

--- Math Checks ---

Total=Reserved+Unalloc+Log: 1394.27 = 1394.27 → True

Reserved=Used+Unused: 226.85 = 226.85 → True

Used=Data+Index: 221.60 = 221.60 → True