[RESOLVED] Inserting a Subtotal

Simple enough right?
Well let me tell ya.. I have a formula that I use to calculate the total amount in the original order and I compare it to what actually shipped. I would like to have a subtotal for the week and one for the month.
The problem is when I try to insert a subtotal option it's grey'd out. Is there another way I can do this?

This the formula that calc the difference;
Sum ({@OrigQtyOrd}, {oehdrhst_sql.ord_no}) - {oehdrhst_sql.tot_sls_amt}

Thanks for your time..

Insert a new formula field that calculate the difference but place it at Detail Section
Then you can add a subtotal for that new formula field

Thanks jggtz, I had to do several other things to make it work but the info you gave helped a great deal.