Length of Substring

Code below sometimes works my problem is that when the subtraction for length is done sometimes it comes back negative and causes query to fail any ideas..

query below is the same just about is the subtraction I need to fix I need to ensure I at least get 1 or 0 what ever makes it not crash

My issue is updating the table to remove the value  "%2F" from the string
its right after the =5299

Any other suggestion will be great

DECLARE @str VARCHAR(8000)

SET @str = 'BT_XYZ=1&referrerID=60081270&campID=5299%2F&BTData=C0216787861617459544B4BBCB6A9BFA59F9B8A89F6EEF7E9E2D9D6C7DE2722BD22EB8&BT_TRF=118447&BT_WAV=20120322&BT_LNK=10151&BT_EML=5543&BT_VAR=1583'
--SET @str = 'BT_XYZ=1&referrerID=60081270&campID=6299&BTData=C0216787861617459544B4BBCB6A9BFA59F9B8A89F6EEF7E9E2D9D6C7DE2722BD22EB8&BT_TRF=118447&BT_WAV=20120322&BT_LNK=10151&BT_EML=5543&BT_VAR=1583'

PRINT @str

SELECT SUBSTRING(@str,CHARINDEX('&campID=',@str),CHARINDEX('&BTData=',@str)-CHARINDEX('&campID=',@str)-3)


SELECT LEN(SUBSTRING(@str,CHARINDEX('&campID=',@str),CHARINDEX('&BTData=',@str)-CHARINDEX('&campID=',@str)))

SELECT REPLACE(@str,(SUBSTRING(@str,CHARINDEX('&campID=',@str),
			   CHARINDEX('&BTData=',@str)-CHARINDEX('&campID=',@str))),(SUBSTRING(@str,CHARINDEX('&campID=',@str),
			   CHARINDEX('&BTData=',@str)-CHARINDEX('&campID=',@str))))

SELECT LEN(SUBSTRING(@str,CHARINDEX('&campID=',@str),CHARINDEX('&BTData=',@str)-CHARINDEX('&campID=',@str)))

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