I think I see your point here. If you want the discount to apply to any item so long as it has a quantity of 25 or more you should use calculations to do this.
You should have two new calculated columns that will check if the quantity is 25 or greater and the type. You will figure the total discount for that line item based on the TotalPrice column. You will have one calculated column for each item type.
This way in the end you can have a blanket discount rule that will give you a total discount based on the subtotal of the new calculated columns. Here is an example of what one of the calculated columns will look like:
abs([Quantity] >= 25 && [ItemType] == 1) *( [TotalPrice] * .10)
This example would check if there is 25 or more of of the item and if it is type 1, of so the column will hold the value of the totalprice for the line item times .10. This will make this column hold 10% of the subtotal for that line item.