space usage on sys.allocation_units and sp_spaceused

It is a known fact that the DMVs dont hold accurate information regarding number of pages and count of rows. However, when you have the stats updated, I can't see why they wouldn't.

I am working on a monitoring tool, want to know disk size of each index and data, etc. Eventually I would like to find the right fill factor, and other things etc.

The space used by my function and the old sp_spaceused differs a little bit on the space usage, but not on record count.

Can you see if there is anything missing in my select?

this is the sp_spaceused (then I convert the numbers in MB):

sp_spaceused 'tblBOrderRelationship'
go

select 318008/1024.00 AS reserved,
140208/1024.00  AS data,
177048/1024.00 AS index_size,
752/1024.00    AS unused

But when I run my select, code below\picture below, I get slightly different figures.

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

SELECT 
    schema_name(t.schema_id) as SchemaName,
    t.NAME AS TableName,
    t.type_desc,
    t.is_ms_shipped,
    t.is_published,
    t.lob_data_space_id,
    t.filestream_data_space_id,
    t.is_replicated,
    t.has_replication_filter,
    t.is_merge_published,
    t.is_sync_tran_subscribed,
    --t.is_filetable,
    i.name as indexName,
    i.type_desc,
    i.is_unique,
    i.is_primary_key,
    i.is_unique_constraint,
    i.fill_factor,
    i.is_padded,


    sum(p.rows)               OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as RowCounts,
    sum(a.total_pages)        OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as TotalPages, 
    sum(a.used_pages)         OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as UsedPages, 
    sum(a.data_pages)         OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as DataPages,

    (sum(a.total_pages)       OVER (PARTITION BY t.OBJECT_ID,i.index_id)  * 8) / 1024 as TotalSpaceMB, 
    (sum(a.used_pages)        OVER (PARTITION BY t.OBJECT_ID,i.index_id)  * 8) / 1024 as UsedSpaceMB, 
    (sum(a.data_pages)        OVER (PARTITION BY t.OBJECT_ID,i.index_id)  * 8) / 1024 as DataSpaceMB
FROM 
    sys.tables t
INNER JOIN      
    sys.indexes i ON t.OBJECT_ID = i.object_id
INNER JOIN 
    sys.partitions p ON i.object_id = p.OBJECT_ID AND i.index_id = p.index_id
INNER JOIN 
    sys.allocation_units a ON p.partition_id = a.container_id
WHERE 
    t.NAME NOT LIKE 'dt%' AND
    i.OBJECT_ID > 255 
AND T.NAME = 'tblBOrderRelationship'

the figures

the bigger picture, including the index names

Now doing some calculations to check the results:

--==================================
-- the figures from sp_spaceused
--==================================
select 318008/1024.00 AS reserved,
140208/1024.00  AS data,
177048/1024.00 AS index_size,
752/1024.00    AS unused

--==================================
-- the figures from my select
--==================================
select 137+61+56+54 AS reserved,
       137 AS data,
       61+56+54 AS index_size

It is not so far off, really, apart the fact I did not calculate the unused space!

What can I do to make this accurate?