How to correctly calculate the weighted average cost of goods after deleting the receipt record?

Question

SQL

Hi everyone
, Can you help me calculate the weighted average cost of goods correctly?
When the goods arrive, it normally counts, but if it deletes the arrival, I can’t calculate it back here correctly ..
the journal table for there is a trigger after insert or after delete
and there are two more tables that update this trigger
, one for storing the balance of GOODSREMAINS and the reference book of goods TGOODS cost fields
Example
average cost = (current balance*current cost price+new.qty+new.price)/(current balance+new.qty)
1)ItemA arrival - 10units - $10 = GOODSREMAINS qty=10 TGOODS GDSAVGCOST=10
2)ItemA sale - 4units - 25$ = GOODSREMAINS qty=6 TGOODS GDSAVGCOST=10
3) ItemA arrival - 10units - $15 = GOODSREMAINS qty=16 TGOODS GDSAVGCOST=13.13
4) ItemA arrival - 10units - $20 = GOODSREMAINS qty=26 TGOODS GDSAVGCOST=15.77
Let's say if you delete the first arrival, how to calculate the cost?

declare variable CURRENT_QTY D_QTY;
declare variable CURRENT_COST D_AMT;
declare variable NEW_COST D_AMT;
begin
 
IF (INSERTING) THEN
  begin
   CURRENT_QTY  =0;
   CURRENT_COST =0;
   NEW_COST     =0;     
 
  select sum(coalesce(qty,0))
    from GOODSREMAINS
   where GOODSID=:NEW.GDSID     
    INTO :CURRENT_QTY;
 
  select coalesce(GDSAVGCOST,0)
    from TGOODS
   where GDSID=:NEW.GDSID     
    INTO :CURRENT_COST;
 
    if (exists(select goodsid from goodsremains where goodsid = :NEW.GDSID and storeid = :NEW.STOREID and boxnumber = :NEW.GDSCOD)) then
        begin
            UPDATE GOODSREMAINS
                   SET GOODSREMAINS.QTY = GOODSREMAINS.QTY + :NEW.RGSTRITEMQTY * :NEW.TRANSTYPE
                        WHERE GOODSREMAINS.GOODSID   = :NEW.GDSID
                               AND GOODSREMAINS.BOXNUMBER = :NEW.GDSCOD
                               AND GOODSREMAINS.STOREID   = :NEW.STOREID;
  end
  else
  begin
    insert into goodsremains (
        goodsid,
        storeid,
        boxnumber,
        qty)
        VALUES (:NEW.GDSID,:NEW.STOREID,:NEW.GDSCOD,:NEW.RGSTRITEMQTY * :NEW.TRANSTYPE);
  end
 
  NEW_COST = (:CURRENT_QTY*:CURRENT_COST+:NEW.RGSTRITEMQTY*:NEW.RGTRITEMPURCHAMT)/(:NEW.RGSTRITEMQTY+:CURRENT_QTY);  
 
  update TGOODS
     set GDSAVGCOST=:NEW_COST
   where GDSID=:NEW.GDSID;
 
end
ELSE IF (DELETING) THEN
   BEGIN
         CURRENT_QTY  =0;
     CURRENT_COST =0;
     NEW_COST     =0;
 
     select sum(coalesce(qty,0))
    from GOODSREMAINS
   where GOODSID=:OLD.GDSID     
    INTO :CURRENT_QTY;
 
  select coalesce(GDSAVGCOST,0)
    from TGOODS
   where GDSID=:OLD.GDSID     
    INTO :CURRENT_COST;
 
     UPDATE GOODSREMAINS
        SET GOODSREMAINS.QTY = GOODSREMAINS.QTY - :OLD.RGSTRITEMQTY * :OLD.TRANSTYPE
      WHERE GOODSREMAINS.GOODSID   = :OLD.GDSID
        AND GOODSREMAINS.BOXNUMBER = :OLD.GDSCOD
        AND GOODSREMAINS.STOREID   = :OLD.STOREID;    
 
 
 
     NEW_COST = ((:CURRENT_QTY-:OLD.RGSTRITEMQTY * :OLD.TRANSTYPE)*:CURRENT_COST-:OLD.RGSTRITEMQTY*:OLD.RGTRITEMPURCHAMT)/:CURRENT_QTY+:OLD.RGSTRITEMQTY;  
 
  update TGOODS
     set GDSAVGCOST=:NEW_COST
   where GDSID=:OLD.GDSID;
   end