Offer discount for ordering certain amount of products - coding help!

I am trying to set up a conditional statement that goes like this: If a customer selects 6 or more products from a list of all possible products, then they will be offered 10% off of their subtotal. I have a field set up to calculate the "Total Quantity" of products ordered and a field for the "Subtotal". I then have a field set up for what the discounted price shoud be, once the 10% is taken off - "Discounted Subtotal. 

I have tried with no success to set this up correctly. Any help/ideas would be greatly appreciated! Thanks!