How is the quote discount calculated based on quote line items?

I would like to know the exact formula the system applies to calculate the Quote Discount starting from quote line items.

I supposed it was a simple average but it is not. I need to know it because I want to be able to build a correct entry criteria at the moment to create an approval wf which triggers when a specific % is applied to a single quote line and not to the entire quote.

The attached image can explain better.