since the grand total is calculated as :
(subtotal + shipping + tax + charges) - discounts
you cannot include the grand total in a discount rule, it would cause an endless loop. because the grand total would change every time the discount where applied.
however, set the discount rul cacluation to a flat rate of:
($this->GetTax() + $this->GetCharges() + $this->GetShipping() + $this->TotalColumn("TotalPrice")) * 0.1
where 0.1 is a 10 percent discount on
subtotal + shipping + tax
not including the discount.