Do you think 15 is about right?

...doesn't end on an even quarter hr (like 9:00:00, 9:15:00, 9:30:00, or 9:45:00), 15 should work. If the auction DOES end on or within a minute of an even quarter hour, you probably should set the lead to low to mid-20 second range.

Let us know how it goes!

Jabbergah