You can try

select *, myCalcField from (your select statement) myInnerSelect

>

	SELECT
>		credits = (SELECT SUM(ac.yamount)
>			FROM account_credits ac
>			WHERE ac.cinmates_id = @cinmates_id) ,
>		debits = (SELECT sum(ad.yamount)
>			from account_debits ad
>			WHERE ad.cinmates_id=@cinmates_id) ,
>		creditedamt = (SELECT sum(ad.ycreditedamt)
>			from account_debits ad
>			WHERE ad.cinmates_id=@cinmates_id )
>

>
>In the above stored proc I would like to add a 4th colum which is balance=credits-debits-creditedamt
>
>Would also appreciate any insights as to other ways to design this select.
>
>TIA