Consider

(+) Using a Create Script to produce an automatically incrementing number. - Sage CRM Hints, Tips and Tricks - Sage CRM - Sage City Community

or

(+) The Sage CRM StoredProc Data Type - Sage CRM Hints, Tips and Tricks - Sage CRM - Sage City Community

I would also see this as a new hidden field on the Company e.g. 'No. Of Master Oppo' (integer)

Then a trigger on the opportunity when it saves. 

DECLARE @Code nVarChar(30)
DELCARE @Proj INT
DELCARE @Type nVarChar(30)
DECLARE @OppoNo nVarChar(30)
DECLARE @ID INT
DELCARE @Comp INT

SELECT
@ID = oppo_opportunityid,
@Comp = oppo_primarycomanyid,
@OppoNo = oppo_ProjectNo (whatever your field is called),
@Type = oppo_type
FROM Inserted

SELECT @Code = comp_ClientCode,
@Proj = CASE WHEN comp_NoOfMasterOppo IS NULL OR comp_NoOfMasterOppo = 0 THEN 1 ELSE comp_NoOfMasterOppo + 1 END
FROM Company WHERE comp_companyid = @Comp

BEGIN
IF @Type = 'Master' AND @OppoNo IS NULL
    BEGIN
    UPDATE Opportunity 
    SET oppo_OppoNo = @Code+'_PO'+CASE(@Proj AS nVarChar(30)) 
    WHERE oppo_opportunity = @ID 

    UPDATE Company
    SET comp_NoOfMasterOppo = @Proj
    WHERE comp_companyid = @Comp
    END
END

brill - thanks for this!

brill - thanks for this!