Have you really read the documentation and the F1 field help, or any of our blogs, documents and wikis here in SCN?
The table that you presented above is far a way from reality. a percentage can go from blank to 99.9 and your from -99.9 to blank
this is to cover both situations early deliveries and late deliveries
the Standard Delivery Date Variance means the it is the maximum number of days a vendor can be late or early to the wanted delivery date, if this value is reached, then he just missed the point in the sense of the word. he does not get any single point for his performance.
You have to ask your business how much days variance they accept before they cancel the order, or at least looking for a new vendor with the next order.
From our point of view everything beyond a week is not at all acceptable and results in a total miss
and in this case you do not need many percentages either in a 5 working day week each day corresponds to 20%