Read my post more carefully and you will see this it the formula I recommended for TotalPrice. The discount amount I state should be:
Your discount rule could be removed entirely if you used that calculation as the totalprice column. That calculation would be the new TotalPrice if you didn't want to use a discount rule and wanted to account for it automatically in the line item.
Anyway your calculation can be:
Any time total Quantity in cart is greater than zero
The quantity measurement is also taken account for in the calculation, so it doesn't need to be restated in the discount rule.