how to sum decimal values in sql server
Ok it's not an anwer just a better formated comment because you are wrong in some way, I has writen the below example.

DECLARE @tcal table(TagIndex smallint,TagName nvarchar(20),Total decimal(18,2)) ;

insert into @tcal
values
 (45,null,284.45)
,(45,null,233.97)
,(45,null,240.17)
,(45,null,81.39)
,(45,null,67.55)
,(45,null,0.00)
,(45,null,71.57)
,(45,null,72.91) ;

select  sum(CAST(Total AS decimal(18,2)))  FROM @tcal where TagIndex&gt;44 ;
select  sum(Total)  FROM @tcal where TagIndex&gt;44 ;


and it works. So you are filtering your data wrong or did some mistake.

My guess is your data is missing the first row because it's tag = 44.

(Remember in SQL > &lt;> >=  =))