SQL Differential Backup Size Prediction / Estimation

/*SQL Differential Backup Size Prediction / Estimation 
 
IF isNULL(object_id('tempdb.dbo.##showFileStats'), 1) <> 1
 DROP TABLE ##showFileStats
 
CREATE TABLE ##showFileStats (
 fileID INT,
 fileGroup INT,
 totalExtents BIGINT,
 usedExtents BIGINT,
 logicalFileName VARCHAR (500), 
 filePath VARCHAR (1000)
)
 
IF isNULL(object_id('tempdb.dbo.##DCM'), 1) <> 1
 DROP TABLE ##DCM
 
CREATE TABLE ##DCM (
 parentObject VARCHAR(5000),
 [object] VARCHAR(5000),
 FIELD VARCHAR (5000),
 VALUE VARCHAR (5000)
)
 
/*we need to get a list of all the files in the database.  each file needs to be looked at*/ 
INSERT INTO ##showFileStats EXEC('DBCC SHOWFILESTATS with tableresults')
 
DECLARE @currentFileID INT,
 @totalExtentsOfFile BIGINT,
 @dbname VARCHAR(100),
 @SQL VARCHAR(200),
 @currentDCM BIGINT,
 @step INT
 
SET @dbname = db_name()
SET @step = 511232
 
DECLARE myCursor SCROLL CURSOR FOR
SELECT fileID, totalExtents 
FROM ##showFileStats
 
OPEN myCursor
FETCH NEXT FROM myCursor INTO @currentFileID, @totalExtentsOfFile
 
/*look at each differential change map page in each data file of the database and put the output into ##DCM*/
WHILE @@FETCH_STATUS = 0 
BEGIN
 
 SET @currentDCM = 6
 WHILE @currentDCM <= @totalExtentsOfFile*8
 BEGIN 
  SET @SQL = 'dbcc page('+ @dbname + ', ' + CAST(@currentFileID AS VARCHAR) + ', ' + CAST(@currentDCM AS VARCHAR) + ', 3) WITH TABLERESULTS'
  INSERT INTO ##DCM EXEC (@SQL)
  SET @currentDCM = @currentDCM + @step
 END
 
 FETCH NEXT FROM myCursor INTO @currentFileID, @totalExtentsOfFile
END
CLOSE myCursor
DEALLOCATE myCursor
 
/*remove all unneeded rows from our results table*/
DELETE FROM ##DCM WHERE VALUE = 'NOT CHANGED' OR parentObject NOT LIKE 'DIFF_MAP%'
--SELECT * FROM ##DCM
 
/*sum the extentTally column*/
SELECT SUM (extentTally) AS totalChangedExtents, SUM(extentTally)/16 AS 'diffPrediction(MB)', SUM(extentTally)/16/1024 AS 'diffPrediction(GB)' 
FROM
 /*create extentTally column*/
 (SELECT extentTally =
 CASE
  WHEN secondChangedExtent > 0 THEN CAST(secondChangedExtent AS BIGINT) - CAST(firstChangedExtent AS BIGINT) + 1
  ELSE 1
 END
 FROM
  /*parse the 'field' column to give us the first and last extents of the range*/
  (SELECT (SUBSTRING(FIELD,(SELECT CHARINDEX(':', FIELD, 0))+1,(CHARINDEX(')', FIELD, 0))-(CHARINDEX(':', FIELD, 0))-1))/8 AS firstChangedExtent,
  secondChangedExtent = 
  CASE 
   WHEN CHARINDEX(':', FIELD, CHARINDEX(':', FIELD, 0)+1) > 0 THEN (SUBSTRING(FIELD,(CHARINDEX(':', FIELD, CHARINDEX(':', FIELD, 0)+1)+1),(CHARINDEX(')', FIELD,CHARINDEX(')', FIELD, 0)+1))-(CHARINDEX(':', FIELD, CHARINDEX(':', FIELD, 0)+1))-1))/8
   ELSE ''
  END 
  FROM ##DCM)parsedFieldColumn)extentTallyColumn